arm64: vdso32: suppress error message for 'make mrproper'

When running the following command without arm-linux-gnueabi-gcc in
one's $PATH, the following warning is observed:

$ ARCH=arm64 CROSS_COMPILE_COMPAT=arm-linux-gnueabi- make -j72 LLVM=1 mrproper
make[1]: arm-linux-gnueabi-gcc: No such file or directory

This is because KCONFIG is not run for mrproper, so CONFIG_CC_IS_CLANG
is not set, and we end up eagerly evaluating various variables that try
to invoke CC_COMPAT.

This is a similar problem to what was observed in
commit dc960bfe

 ("h8300: suppress error messages for 'make clean'")

Reported-by: Lucas Henneman <henneman@google.com>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20211019223646.1146945-4-ndesaulniers@google.com
Signed-off-by: Will Deacon <will@kernel.org>