x86: vdso: Fix leaky vdso linker with CC=clang.

x86: vdso: Fix leaky vdso linker with CC=clang.

The vdso{32,64}.so can fail to build when CC=clang when clang tries to
find a suitable GCC toolchain to link these libraries with.

/usr/bin/ld: arch/x86/entry/vdso/vclock_gettime.o: access beyond end of merged section (782)

This happens because the host environment leaked into the CROSS_COMPILE
environment due to the way clang searches for suitable GCC toolchains.

Most of the time this goes unnoticed because the host linker is new
enough to work anyway, but on this particular machine it was not.

Extract the needed --target and --gcc-toolchain flags added in the top
level Makefile from KBUILD_CFLAGS.

Bug: 63889157
Change-Id: If7d4097d1d2eaf95f18d0295483bde8792a06844
Signed-off-by: Alistair Strachan <astrachan@google.com>