Simplify __ARM_ARCH__ definition.

OpenSSL's assembly files have a few places where we condition code on
__ARM_ARCH__, the minimum target ARM revision. It currently only
controls some pre-ARMv7 code. This symbol has, from what I can tell, the
same semantics as __ARM_ARCH, defined in Arm C Language Extensions, and
added in GCC 4.8 and Clang 3.2:
https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=9e94a7fc5ab770928b9e6a2b74e292d35b4c94da;hp=25bab91e017eb1d6d93117f3da96fa9b43703190
e98c4dbd1e

Those are over nine years old, so drop all the fallback code. Also fix
arm_arch.h to be includable on non-ARM platforms. Some tools expect all
public headers to be cleanly includable and arm_arch.h being "public"
was getting in the way (see cl/416881417).

Interestingly, arm_arch.h previously only computed __ARM_ARCH__ for
__GNUC__ and Clang doesn't define __GNUC__ on Windows. That means we
actually weren't defining __ARM_ARCH__ for Windows. But none of the
aarch64 assembly has __ARM_ARCH__-gated code, so it works out. If it
ever does, that CL smooths that over. I've gated the
__ARM_(MAX_)_ARCH__ bits on __ASSEMBLER__ to avoid breaking no-asm
Windows/aarch64 builds on MSVC. There aren't any uses in C.

Update-Note: ARM assembly now requires the compiler define __ARM_ARCH.
This is not expected to break Clang or GCC from the last 8 or 9 years.

Change-Id: Id45e95406edeecf8dda11dce9e82418516e9de1f
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/50849
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/arm_arch.h

@@ -53,49 +53,12 @@
 #ifndef OPENSSL_HEADER_ARM_ARCH_H
 #define OPENSSL_HEADER_ARM_ARCH_H
 
-#if !defined(__ARM_ARCH__)
-# if defined(__CC_ARM)
-#  define __ARM_ARCH__ __TARGET_ARCH_ARM
-#  if defined(__BIG_ENDIAN)
-#   define __ARMEB__
-#  else
-#   define __ARMEL__
-#  endif
-# elif defined(__GNUC__)
-#  if defined(__aarch64__)
-#    define __ARM_ARCH__ 8
-  // Why doesn't gcc define __ARM_ARCH__? Instead it defines
-  // bunch of below macros. See all_architectires[] table in
-  // gcc/config/arm/arm.c. On a side note it defines
-  // __ARMEL__/__ARMEB__ for little-/big-endian.
-#  elif	defined(__ARM_ARCH)
-#    define __ARM_ARCH__ __ARM_ARCH
-#  elif	defined(__ARM_ARCH_8A__)
-#    define __ARM_ARCH__ 8
-#  elif	defined(__ARM_ARCH_7__)	|| defined(__ARM_ARCH_7A__)	|| \
-	defined(__ARM_ARCH_7R__)|| defined(__ARM_ARCH_7M__)	|| \
-	defined(__ARM_ARCH_7EM__)
-#   define __ARM_ARCH__ 7
-#  elif	defined(__ARM_ARCH_6__)	|| defined(__ARM_ARCH_6J__)	|| \
-	defined(__ARM_ARCH_6K__)|| defined(__ARM_ARCH_6M__)	|| \
-	defined(__ARM_ARCH_6Z__)|| defined(__ARM_ARCH_6ZK__)	|| \
-	defined(__ARM_ARCH_6T2__)
-#   define __ARM_ARCH__ 6
-#  elif	defined(__ARM_ARCH_5__)	|| defined(__ARM_ARCH_5T__)	|| \
-	defined(__ARM_ARCH_5E__)|| defined(__ARM_ARCH_5TE__)	|| \
-	defined(__ARM_ARCH_5TEJ__)
-#   define __ARM_ARCH__ 5
-#  elif	defined(__ARM_ARCH_4__)	|| defined(__ARM_ARCH_4T__)
-#   define __ARM_ARCH__ 4
-#  else
-#   error "unsupported ARM architecture"
-#  endif
-# endif
-#endif
+// arm_arch.h contains symbols used by ARM assembly, and the C code that calls
+// it. It is included as a public header to simplify the build, but is not
+// intended for external use.
 
-// Even when building for 32-bit ARM, support for aarch64 crypto instructions
-// will be included.
-#define __ARM_MAX_ARCH__ 8
+#if defined(__ARMEL__) || defined(_M_ARM) || defined(__AARCH64EL__) || \
+    defined(_M_ARM64)
 
 // ARMV7_NEON is true when a NEON unit is present in the current CPU.
 #define ARMV7_NEON (1 << 0)
@@ -117,6 +80,23 @@
 
 #if defined(__ASSEMBLER__)
 
+// We require the ARM assembler provide |__ARM_ARCH| from Arm C Language
+// Extensions (ACLE). This is supported in GCC 4.8+ and Clang 3.2+. MSVC does
+// not implement ACLE, but we require Clang's assembler on Windows.
+#if !defined(__ARM_ARCH)
+#error "ARM assembler must define __ARM_ARCH"
+#endif
+
+// __ARM_ARCH__ is used by OpenSSL assembly to determine the minimum target ARM
+// version.
+//
+// TODO(davidben): Switch the assembly to use |__ARM_ARCH| directly.
+#define __ARM_ARCH__ __ARM_ARCH
+
+// Even when building for 32-bit ARM, support for aarch64 crypto instructions
+// will be included.
+#define __ARM_MAX_ARCH__ 8
+
 // Support macros for
 //   - Armv8.3-A Pointer Authentication and
 //   - Armv8.5-A Branch Target Identification
@@ -233,6 +213,8 @@
 .popsection;
 #endif
 
-#endif  /* defined __ASSEMBLER__ */
+#endif  // __ASSEMBLER__
+
+#endif  // __ARMEL__ || _M_ARM || __AARCH64EL__ || _M_ARM64
 
 #endif  // OPENSSL_HEADER_ARM_ARCH_H