amd/addrlib: limit fastcall/regparm to GCC i386

amd/addrlib: limit fastcall/regparm to GCC i386

The use of regparm causes an error on arm/arm64 builds with clang.
fastcall is allowed, but still throws a warning. As both options only
have effect on 32-bit x86 builds, limit them to that case.

v2: keep the __i386__ within GCC (Nicolai)

Cc: 13.0 <mesa-stable@lists.freedesktop.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Nicolai Hähnle <nhaehnle@gmail.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Rob Herring <robh@kernel.org>