[M68k] Correctly emit non-pic relocations (#89863)

The m68k backend will always emit external calls (including libcalls)
with
PC-relative PLT relocations, even when in non-pic mode or -fno-plt is
used.

This is unexpected, as other function calls are emitted with absolute
addressing, and a static code modes suggests that there is no PLT. It
also
leads to a miscompilation where the call instruction emitted expects an
immediate address, while the relocation emitted for that instruction is
PC-relative.

This miscompilation can even be seen in the default C function in
godbolt:
https://godbolt.org/z/zEoazovzo

Fix the issue by classifying external function references based upon the
pic
mode. This triggers a change in the static code model, making it more in
line
with the expected behaviour and allowing use of this backend in more
bare-metal
situations where a PLT does not exist.

The change avoids the issue where we emit a PLT32 relocation for an
absolute
call, and makes libcalls and other external calls use absolute
addressing modes
when a static code model is desired.

Further work should be done in instruction lowering and validation to
ensure
that miscompilations of the same type don't occur.