Add Asylo platform support.

This patch enables the open source portable enclave framework
[Asylo](https://github.com/google/asylo) to use gRPC within its
toolchain.

The Asylo EKEP handshaker does its own end-to-end testing by reusing
gRPC's own end2end testing targets. To do so, it requires those targets
to be visible.

Signed-off-by: Dionna Amalie Glaze <dionnaglaze@google.com>

include/grpc/impl/codegen/port_platform.h

@@ -186,6 +186,26 @@
 #else /* musl libc */
 #define GPR_MUSL_LIBC_COMPAT 1
 #endif
+#elif defined(__ASYLO__)
+#define GPR_ARCH_64 1
+#define GPR_CPU_POSIX 1
+#define GPR_GCC_TLS 1
+#define GPR_PLATFORM_STRING "asylo"
+#define GPR_GCC_SYNC 1
+#define GPR_POSIX_SYNC 1
+#define GPR_POSIX_STRING 1
+#define GPR_POSIX_LOG 1
+#define GPR_POSIX_TIME 1
+#define GPR_POSIX_ENV 1
+#define GPR_ASYLO 1
+#define GRPC_POSIX_SOCKET 1
+#define GRPC_POSIX_SOCKETADDR
+#define GRPC_POSIX_SOCKETUTILS 1
+#define GRPC_TIMER_USE_GENERIC 1
+#define GRPC_POSIX_NO_SPECIAL_WAKEUP_FD 1
+#define GRPC_POSIX_WAKEUP_FD 1
+#define GRPC_ARES 0
+#define GPR_NO_AUTODETECT_PLATFORM 1
 #elif defined(__APPLE__)
 #include <Availability.h>
 #include <TargetConditionals.h>

test/core/end2end/generate_tests.bzl

@@ -380,6 +380,7 @@ def grpc_end2end_tests():
             "end2end_tests.h",
         ],
         language = "C++",
+        visibility = ["//visibility:public"],
         deps = [
             ":cq_verifier",
             ":ssl_test_data",