libstdc++: Use GCC's predefined macro for endianness [PR104080]

libstdc++: Use GCC's predefined macro for endianness [PR104080]

Instead of hardcoded preprocessor conditionals with explicit target
checks, just rely on the fact that __BYTE_ORDER__ is always defined by
GCC.

libstdc++-v3/ChangeLog:

	PR libstdc++/104080
	* src/c++17/fast_float/LOCAL_PATCHES: Update.
	* src/c++17/fast_float/fast_float.h (FASTFLOAT_IS_BIG_ENDIAN):
	Define in terms of __BYTE_ORDER__.