core: fix x86 PIC code compilation

This bug was triggered by Buildroot autobuilders [1,2], causing this
kind of failures [3,4]:

  [ 14%] Building CXX object modules/core/CMakeFiles/opencv_core.dir/src/system.cpp.o
  /home/test/autobuild/instance-0/output/build/opencv-2.4.10/modules/core/src/system.cpp: In function '(static initializers for /home/test/autobuild/instance-0/output/build/opencv-2.4.10/modules/core/src/system.cpp)':
  /home/test/autobuild/instance-0/output/build/opencv-2.4.10/modules/core/src/system.cpp:280:10: error: inconsistent operand constraints in an 'asm'
  make[3]: *** [modules/core/CMakeFiles/opencv_core.dir/src/system.cpp.o] Error 1

[1] http://buildroot.org/
[2] http://autobuild.buildroot.org/
[3] http://autobuild.buildroot.org/?reason=opencv-2.4.10
[4] http://autobuild.buildroot.org/results/483/4838285b25d6293a5cf0bb9eadd5040a7c75d766/build-end.log

Signed-off-by: Samuel Martin <s.martin49@gmail.com>

modules/core/src/system.cpp

@@ -267,14 +267,17 @@ struct HWFeatures
          : "cc"
         );
         #else
+        // We need to preserve ebx since we are compiling PIC code.
+        // This means we cannot use "=b" for the 2nd output register.
         asm volatile
         (
          "pushl %%ebx\n\t"
          "movl $7,%%eax\n\t"
          "movl $0,%%ecx\n\t"
          "cpuid\n\t"
+         "movl %%ebx,%1\n\t"
          "popl %%ebx\n\t"
-         : "=a"(cpuid_data[0]), "=b"(cpuid_data[1]), "=c"(cpuid_data[2]), "=d"(cpuid_data[3])
+         : "=a"(cpuid_data[0]), "=r"(cpuid_data[1]), "=c"(cpuid_data[2]), "=d"(cpuid_data[3])
          :
          : "cc"
         );