fat_init violates host ABI on Win64

[Nicolas Hake]

Torbjörn Granlund wrote:
> Where do we (unconditionally) rely on variable-length arrays?

mpn/generic/sqrlo_basecase.c line 153, where SQRLO_BASECASE_ALLOC is a 
non-constant expression by ultimately referencing CPUVEC_THRESHOLD if 
fat.h is included.
mpn/generic/sqr_basecase.c line 170 has the same issue via 
SQR_TOOM2_THRESHOLD.

Both macros are constant if fat.h is not included.

Nicolas