[libunwind] Fix wrong endianness check in Unwind-EHABI

[speidy]

Summary

The ARM specific code was trying to determine endianness using the
__LITTLE_ENDIAN__ macro which is not guaranteed to be defined.
When not defined, it makes libunwind to build the big-endian code even
when the compiler builds for a little-endian target.

This issue leads libunwind to crash with SIGSEGV during stack unwinding when
it built for ARM by using musl-gcc toolchain (from http://musl.cc) and breaks exception
handling.

Switched into a more hermetic check which should also raise a
compile-time error in case endianness could not be determined.

[mstorsjo]

LGTM

[speidy]

Can you merge?

[compnerd]

This seems fine with the minor thing that the original check was not incorrect, merely not as portable, __BYTE_ORDER__ is more portable.

[compnerd]

Merged as rG68c50708d1f2b9aee3f10ec710df0b1387f701e5.

Thanks for the portability fix!

libunwind/src/Unwind-EHABI.cpp

 #include "unwind.h"
 
 namespace {
 
 // Strange order: take words in order, but inside word, take from most to least
 // signinficant byte.
 uint8_t getByte(const uint32_t* data, size_t offset) {
   const uint8_t* byteData = reinterpret_cast<const uint8_t*>(data);
-#ifdef __LITTLE_ENDIAN__
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
   return byteData[(offset & ~(size_t)0x03) + (3 - (offset & (size_t)0x03))];
-#else
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
   return byteData[offset];
+#else
+#error "Unable to determine endianess"
 #endif
 }
 
 const char* getNextWord(const char* data, uint32_t* out) {
   *out = *reinterpret_cast<const uint32_t*>(data);
   return data + 4;
 }
 
case _UVRSC_VFP: {
         return _UVRSR_FAILED;
       }
       // For _UVRSD_VFPX, we're assuming the data is stored in FSTMX "standard
       // format 1", which is equivalent to FSTMD + a padding word.
       for (uint32_t i = first; i < end; ++i) {
         // SP is only 32-bit aligned so don't copy 64-bit at a time.
         uint64_t w0 = *sp++;
         uint64_t w1 = *sp++;
-#ifdef __LITTLE_ENDIAN__
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
         uint64_t value = (w1 << 32) | w0;
-#else
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
         uint64_t value = (w0 << 32) | w1;
+#else
+#error "Unable to determine endianess"
 #endif
         if (_Unwind_VRS_Set(context, regclass, i, representation, &value) !=
             _UVRSR_OK)
           return _UVRSR_FAILED;
       }
       if (representation == _UVRSD_VFPX)
         ++sp;
       return _Unwind_VRS_Set(context, _UVRSC_CORE, UNW_ARM_SP, _UVRSD_UINT32,