efi/libstub/x86: Avoid thunking for native firmware calls

We use special wrapper routines to invoke firmware services in the
native case as well as the mixed mode case. For mixed mode, the need
is obvious, but for the native cases, we can simply rely on the
compiler to generate the indirect call, given that GCC now has
support for the MS calling convention (and has had it for quite some
time now). Note that on i386, the decompressor and the EFI stub are not
built with -mregparm=3 like the rest of the i386 kernel, so we can
safely allow the compiler to emit the indirect calls here as well.

So drop all the wrappers and indirection, and switch to either native
calls, or direct calls into the thunk routine for mixed mode.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Borislav Petkov <bp@alien8.de>
Cc: James Morse <james.morse@arm.com>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: https://lkml.kernel.org/r/20191224151025.32482-14-ardb@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>