loader/efi: Move ARM64 linux loader to common code

ARM64 linux loader code is written in such a way that it can be reused
across different architectures without much change. Move it to common
code so that RISC-V doesn't have to define a separate loader.

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/Makefile.core.def

@@ -1833,9 +1833,9 @@ module = {
   sparc64_ieee1275 = loader/sparc64/ieee1275/linux.c;
   ia64_efi = loader/ia64/efi/linux.c;
   arm_coreboot = loader/arm/linux.c;
-  arm_efi = loader/arm64/linux.c;
+  arm_efi = loader/efi/linux.c;
   arm_uboot = loader/arm/linux.c;
-  arm64 = loader/arm64/linux.c;
+  arm64 = loader/efi/linux.c;
   riscv32 = loader/riscv/linux.c;
   riscv64 = loader/riscv/linux.c;
   emu = loader/emu/linux.c;