arm64: vdso: use $(LD) for linking .so

When building for Android using CC for linking produces an error
because its clang calls the host linker aarch64-linux-gnu-ld
if available but the build tools prohibit the use of host
linkers.

Vanilla nowadays also uses LD since 691efbedc60d2a7364a90e38882fc762f06f52c4
but the same approach did not work because of some missing changes in
Makefile.lib and bad interactions with the compile steps.

Change-Id: I5e42f300a0523f710dccc0e73138d21ccbf55e96