This avoids AArch64 legality rules depending on libcall
availability.
ARM, AArch64, and X86 all had custom lowering of fsincos which
all were just to emit calls to sincos_stret / sincosf_stret. This
messes with the cost heuristics around legality, because really
it's an expand/libcall cost and not a favorable custom.
This is a bit ugly, because we're emitting code trying to match the
C ABI lowered IR type for the aggregate return type. This now also
gives an easy way to lift the unhandled x86_32 darwin case, since
ARM already handled the return as sret case.
831e79adff