sd-boot: Get rid of uefi_call_wrapper

sd-boot: Get rid of uefi_call_wrapper

The uefi_call_wrapper exists to convert to the right calling convention
and presumably predates compilers that can do so natively. The only
architecture where this is even needed is x86_64.
But because we are building with GNU_EFI_USE_MS_ABI defined, the
EFIAPI macro tells the compiler to use the right calling convention
for EFI functions. Our shim callback (which is called by EFI itself)
already relies on this.

This also adds a safety check to make se we are compiling with
GNU_EFI_USE_MS_ABI defined and also adds it to the compiler args
unconditionally. It is only used with x86_64 anyways, so it should
be fine to do so. EFI_FUNCTION_WRAPPER is unused in gnu-efi, so
it is dropped.