AMD64: Set fp_ret_offset

[mborgerson]

AMD64 SysV returns float/double in xmm0 (offset 224). Without this, the decompiler's AIL converter never created fp_ret_expr for call statements, so FP return values from calls were lost.

[rhelmot]

This is probably good to merge as-is, but the calling convention stuff in archinfo is a bit of a hackjob since it should really be handled on a per-calling-convention basis.

[mborgerson]

should really be handled on a per-calling-convention basis

Agree

[ltfish]

should really be handled on a per-calling-convention basis

It requires a much bigger refactor especially in the static analysis realm.

[twizmwazin]

It requires a much bigger refactor especially in the static analysis realm.

Can we replace the use of archinfo constants with the equivalent value from the default calling convention?

[mborgerson]

I don't plan to address the architectural issues or refactoring in this PR, we'll handle it in a follow up

[ltfish]

Can we just merge this PR? I think it's good enough as it is.