meson.bbclass: Point to llvm-config from native sysroot

Default search in meson would grok /usr/bin for llvm-config and if found
will use it, which might add wrong paths into cflags/ldflags, since we
depend on llvm-native when building gallium support ( thats when
llvm-config is effective), its better to point llvm-config into native
sysroot so it can add correct paths into compiler/linker cmdline

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>