align: use __builtin_unreachable on Cray.
Cray doesn't support __builtin_assume_aligned.  This isn't quite
enough to get craycc working since we also need some changes in
Hedley which I'm working on.
HEDLEY_SUNPRO_VERSION_CHECK(5,13,0) || \
HEDLEY_TINYC_VERSION_CHECK(0,9,24) || \
HEDLEY_PGI_VERSION_CHECK(19,10,0) || \
+ HEDLEY_CRAY_VERSION_CHECK(10,0,0) || \
HEDLEY_TI_ARMCL_VERSION_CHECK(16,9,0) || \
HEDLEY_TI_CL2000_VERSION_CHECK(16,9,0) || \
HEDLEY_TI_CL6X_VERSION_CHECK(8,0,0) || \
