UPSTREAM: arm64: Default to building compat vDSO with clang when CONFIG_CC_IS_CLANG

Rather than force the use of GCC for the compat cross-compiler, instead
extract the target from CROSS_COMPILE_COMPAT and pass it to clang if the
main compiler is clang.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
(cherry picked from commit 24ee01a927bfe56c66429ec4b1df6955a814adc8)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 154668398
Change-Id: I66ffadbc63519d9d97557335a4050d8c33233b5b