libclc: do native build of prepare_builtins in a fresh environment

libclc: do native build of prepare_builtins in a fresh environment

The environment has variables like LDFLAGS set which CMake will try and
use, so until our cmake class is comprehensive enough to nuke those in
the native toolchain file (if that is possible) do a build inside a
clean (apart from PATH) environment to avoid cross-contamination.

As noticed by Khem Raj, this can fail in nativesdk builds as the target
flags will be passed to the native link resulting in failures like:

  Requesting program interpreter: /usr/local/oe-sdk-hardcoded-buildpath/sysroots/x86_64-yoesdk-linux/lib/ld-linux-x86-64.so.2

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>