FROMLIST: arm64: vdso32: Use full path to Clang instead of relying on PATH

Currently, in order to build the compat VDSO with Clang, this format
has to be used:

  PATH=${BIN_FOLDER}:${PATH} make CC=clang

Prior to the addition of this file, this format would also be
acceptable:

  make CC=${BIN_FOLDER}/clang

This is because the vdso32 Makefile uses cc-name instead of CC. After
this path, CC will still evaluate to clang for the first case as
expected but now the second case will use the specified Clang, rather
than the host's copy, which may not be compatible as shown below.

/usr/bin/as: unrecognized option '-mfloat-abi=soft'
clang-6.0: error: assembler command failed with exit code 1

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
(cherry picked from https://patchwork.kernel.org/patch/10419665)
Bug: 80184372
Change-Id: If90a5a4edbc2b5883b4c78161081ebeafbebdcde