Closes #5643

This also removes the need to pass noexecstack to gcc, but that wasn't
actually working anymore.

This doesn't matter now since we use an out pointer for return
values but it's sure to show up mysteriously someday.

If Rust code made use of return values then using %eax here would clobber it

Had to bump the min stack size in some of the tests from 256 bytes
to 1024 bytes. Not sure why yet.

The DW_CFA_val_offset_sf instruction doesn't seem to work on mac,
even after implementing it in the llvm-mc assembler, so now
I'm looking for a different way to communicate the stack pointer
adjustment to the unwinder.

LLVM's assembler understands .cfi pseudo-ops on the mac, which we need in
order to generate frames that can be DWARF-unwound

upcall_call_shim_on_c_stack does not handle return values

Currently broken everywhere else

Each crate will have its own copy of this function, which should not be
exported.