UPSTREAM: arm64: build vdso without libgcov

(cherry pick from commit 54309784)

On a cross-toolchain without glibc support, libgcov may not be
available, and attempting to build an arm64 kernel with GCOV
enabled then results in a build error:

/home/arnd/cross-gcc/lib/gcc/aarch64-linux/5.2.1/../../../../aarch64-linux/bin/ld: cannot find -lgcov

We don't really want to link libgcov into the vdso anyway, so
this patch just disables GCOV in the vdso directory, just as
we do for most other architectures.

Change-Id: I98e4a9f1339c4668a8dca7b26cd88c3b424c536c
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Bug: 20045882
Bug: 19198045
hange-Id: Ia9431e825918fbdf07f9e62f893449451fe14327