arm64: Adjust EFI libstub object include logic

Commit f4f75ad5 ("efi: efistub: Convert into static library")
introduced a static library for EFI stub, libstub.

The EFI libstub directory is referenced by the kernel build system via
a obj subdirectory rule in:
drivers/firmware/efi/Makefile

Unfortunately, arm64 also references the EFI libstub via:
libs-$(CONFIG_EFI_STUB) += drivers/firmware/efi/libstub/

If we're unlucky, the kernel build system can enter libstub via two
simultaneous threads resulting in build failures such as:

fixdep: error opening depfile: drivers/firmware/efi/libstub/.efi-stub-helper.o.d: No such file or directory
scripts/Makefile.build:257: recipe for target 'drivers/firmware/efi/libstub/efi-stub-helper.o' failed
make[1]: *** [drivers/firmware/efi/libstub/efi-stub-helper.o] Error 2
Makefile:939: recipe for target 'drivers/firmware/efi/libstub' failed
make: *** [drivers/firmware/efi/libstub] Error 2
make: *** Waiting for unfinished jobs....

This patch adjusts the arm64 Makefile to reference the compiled library
explicitly (as is currently done in x86), rather than the directory.

Fixes: f4f75ad5

 efi: efistub: Convert into static library
Signed-off-by: Steve Capper <steve.capper@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>