linux-yocto: depend on libgcc for aarch64

linux-yocto: depend on libgcc for aarch64

Make aarch aarch64 kernel depend on libgcc.

In arch/arm64/Makefile, it adds LIBGCC to libs-y:

LIBGCC          := $(shell $(CC) $(KBUILD_CFLAGS) -print-libgcc-file-name)
libs-y          += $(LIBGCC)

In file Makefile in top directory, libs-y is assigned to to var
KBUILD_VMLINUX_MAIN. It uses script link-vmlinux.sh to link vmlinux.o,
and when execute function vmlinux_link() in link-vmlinux.sh,
KBUILD_VMLINUX_MAIN is passed to ${LD}.

If build without libgcc, the value of LIBGCC is just libgcc.a without
parent directory. linux-yocto fails to build:

|   LD      vmlinux.o
| aarch64-poky-linux-ld.bfd: cannot find libgcc.a: No such file or directory

Add libgcc to aarch64 kernel dependency.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>