sanitizer: Fix endianness checks for gcc

Summary:
__BIG_ENDIAN__ and __LITTLE_ENDIAN__ are not supported by gcc, which
eg. for ubsan Value::getFloatValue will silently fall through to
the little endian branch, breaking display of float values by ubsan.
Use __BYTE_ORDER__ == __ORDER_BIG/LITTLE_ENDIAN__ as the condition
instead, which is supported by both clang and gcc.

Noticed while porting ubsan to s390x.

Patch by Marcin Kościelnicki!

Differential Revision: http://reviews.llvm.org/D17660

llvm-svn: 263077