arm64: vdso32: Fix '--prefix=' value for newer versions of clang

Newer versions of clang only look for $(COMPAT_GCC_TOOLCHAIN_DIR)as [1],
rather than $(COMPAT_GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE_COMPAT)as,
resulting in the following build error:

$ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \
CROSS_COMPILE_COMPAT=arm-linux-gnueabi- LLVM=1 O=out/aarch64 distclean \
defconfig arch/arm64/kernel/vdso32/
...
/home/nathan/cbl/toolchains/llvm-binutils/bin/as: unrecognized option '-EL'
clang-12: error: assembler command failed with exit code 1 (use -v to see invocation)
make[3]: *** [arch/arm64/kernel/vdso32/Makefile:181: arch/arm64/kernel/vdso32/note.o] Error 1
...

Adding the value of CROSS_COMPILE_COMPAT (adding notdir to account for a
full path for CROSS_COMPILE_COMPAT) fixes this issue, which matches the
solution done for the main Makefile [2].

[1]: https://github.com/llvm/llvm-project/commit/3452a0d8c17f7166f479706b293caf6ac76ffd90
[2]: https://lore.kernel.org/lkml/20200721173125.1273884-1-maskray@google.com/



Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Cc: stable@vger.kernel.org
Link: https://github.com/ClangBuiltLinux/linux/issues/1099
Link: https://lore.kernel.org/r/20200723041509.400450-1-natechancellor@gmail.com


Signed-off-by: Will Deacon <will@kernel.org>
[dl: Backported to 4.14, depends on commit 38253a0ed057c49dde77588eef05fdcb4008ce0b
     ("vdso32: Invoke clang with correct path to GCC toolchain")
     from the Pixel 4 kernel]
Signed-off-by: Danny Lin <danny@kdrag0n.dev>