efi: Add mixed-mode thunk recipe for GetMemoryAttributes

EFI mixed mode on x86 requires a recipe for each protocol method or
firmware service that takes u64 arguments by value, or returns pointer
or 'native int' (UINTN) values by reference (e.g,, through a void ** or
unsigned long * parameter), due to the fact that these types cannot be
translated 1:1 between the i386 and MS x64 calling conventions.

So add the missing recipe for GetMemoryAttributes, which is not actually
being used yet on x86, but the code exists and can be built for x86 so
let's make sure it works as it should.

Cc: Evgeniy Baskov <baskov@ispras.ru>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>