[aarch64][libstdc++] Use __ARM_BIG_ENDIAN instead of __AARCH64EB__ in opt_random.h

[aarch64][libstdc++] Use __ARM_BIG_ENDIAN instead of __AARCH64EB__ in opt_random.h

As has been spotted at https://gcc.gnu.org/ml/gcc-patches/2017-12/msg01289.html
we check the __AARCH64EB__ macro for aarch64 big-endian
detection in config/cpu/aarch64/opt/ext/opt_random.h.
That works just fine with GCC but the standardised ACLE[1] macro
for that purpose is __ARM_BIG_ENDIAN so there is a possibility
that non-GCC compilers that include this header are not aware
of this predefine.

So this patch changes the use of __AARCH64EB__ to
the more portable __ARM_BIG_ENDIAN.

Tested on aarch64-none-elf and aarch64_be-none-elf.

Preapproved by Jeff at https://gcc.gnu.org/ml/gcc-patches/2017-12/msg01326.html

	* config/cpu/aarch64/opt/ext/opt_random.h (__VEXT): Check
	__ARM_BIG_ENDIAN instead of __AARCH64EB__.

From-SVN: r255857