buildtools-tarball: set pkg-config search path

The buildtools-tarball includes a native sysroot with .pc files, and
in the case of buildtools-extended-tarball a pkg-config binary too.

If we're using the host pkg-config then it doesn't know to search in the
native sysroot.  If we're using our pkg-config then it searches in the
build-time prefix and not the actual SDK installation location.

Neither of these are correct, so set PKG_CONFIG_LIBDIR to search:
- The native sysroot
- The host pkg-config's default search path, if present
- Falling back to /usr/lib/pkgconfig, if not

In an ideal world this would be handled by the generic toolchain script,
but that is slightly more involved.

[ YOCTO #15007 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 8f768a3f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>