This patch makes the STG->Cmm backend avoid saving foreign call target
to local when there are no caller-save GlobalRegs.

Since 321941a8, when we lower a
foreign call, we unconditionally save the foreign call target to a
temporary local first, then rely on cmmSink to clean it up later,
which only happens with -fcmm-sink (implied by -O) and not in
unoptimized code.

And this is troublesome for the wasm backend NCG, which needs to infer
a foreign call target symbol's type signature from the Cmm call site.
Previously, the NCG has been emitting incorrect type signatures for
unoptimized code, which happens to work with `wasm-ld` most of the
time, but this is never future-proof against upstream toolchain
updates, and it causes horrible breakages when LTO objects are
included in linker input. Hence this patch.

(cherry picked from commit 8dd8a076)
(cherry picked from commit 2b2a3db8)