BaseTools/tools_def AARCH64: avoid SIMD registers in XIP code

BaseTools/tools_def AARCH64: avoid SIMD registers in XIP code

XIP code may execute with the MMU off, in which case all memory accesses
should be strictly aligned to their size. Some versions of GCC violate
this restriction even when -mstrict-align is passed, when performing
loads and stores that involve SIMD registers. This is clearly a bug in
the compiler, but we can easily work around it by avoiding SIMD registers
altogether when building code that may execute in such a context. So add
-mgeneral-regs-only to the AARCH64 XIP CC flags.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>