efi/x86: Rely on compiler to emit MS ABI calls

When EFI support was added to the x86_64 port originally, GCC did not
implement support for emitting procedure calls using the MS calling
convention, which deviates from the SysV one used in Linux.

However, this support has been added a long time ago, and the EFI stub
(which runs in a different execution context) has already been updated
to simply rely on the __attribute__((ms_abi)) annotations of the EFI
function pointers, resulting in both GCC and Clang doing the right
thing, as long as the correct stack alignment is being used.

The EFI stub runs on the stack provided by the firmware, and maintains
16 byte alignment internally, but the core x86_64 code does not, so
explicit stack realignment is needed, and this has been dealt with in
a previous patch. So the asm wrapper has become redundant, and can be
dropped.

Note that one of the EFI runtime services returns void so a trick is
needed to make the compiler ignore the return value in that case, or an
error will be triggered.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>