arm64: vdso: use $(LD) instead of $(CC) to link VDSO

We use $(LD) to link vmlinux, modules, decompressors, etc.

VDSO is the only exceptional case where $(CC) is used as the linker
driver, but I do not know why we need to do so. VDSO uses a special
linker script, and does not link standard libraries at all.

I changed the Makefile to use $(LD) rather than $(CC). I tested this,
and VDSO worked for me.

Users will be able to use their favorite linker (e.g. lld instead of
of bfd) by passing LD= from the command line.

My plan is to rewrite all VDSO Makefiles to use $(LD), then delete
cc-ldoption.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>