[X86] Remove regcall calling convention from fp-strict-scalar.ll. Add 32-bit...

[X86] Remove regcall calling convention from fp-strict-scalar.ll. Add 32-bit and 64-bit check prefixes.

The regcall was making 32-bit mode pass things in xmm registers
which made 32-bit and 64-bit more similar. But I have upcoming
patches that require them to be separated anyway.