ARM execinfo fixes (awslabs#598)

* look for arm when looking for execinfo

Author: Justin Boswell

cmake/AwsFeatureTests.cmake

@@ -45,7 +45,8 @@ int main() {
     __asm__ __volatile__(\"\":\"=r\"(foo):\"r\"(bar):\"memory\");
 }" AWS_HAVE_GCC_INLINE_ASM)
 
-if(NOT LEGACY_COMPILER_SUPPORT)
+string(REGEX MATCH "^(aarch64|arm)" ARM_CPU ${CMAKE_SYSTEM_PROCESSOR})
+if(NOT LEGACY_COMPILER_SUPPORT OR ARM_CPU)
     check_c_source_compiles("
     #include <execinfo.h>
     int main() {