[Support/BLAKE3] Fix error when building llvm for big endian AArch64 host

BLAKE3 implementation does not support using arm neon on big-endian hosts: see
blake3_neon.c. Setting `BLAKE3_USE_NEON` to 1 by default for all AArch64
hosts broke builds for big endian hosts. This patch fixes the behavior
by introducing an additional check against `__ARM_BIG_ENDIAN` before
setting `BLAKE3_USE_NEON`.

Differential Revision: https://reviews.llvm.org/D159156