builtin_compiler/build: Avoid using libtool if cross compiling

builtin_compiler/build: Avoid using libtool if cross compiling

Adds the dependencies of builtin_compiler as sources when cross
compiling instead of using libtool to share compilation with src/glsl.
The builtin_compiler executable is built for the host when cross
compiling so it doesn't make sense to share compilation with src/glsl
built for the target in this case.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=44618
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Jonathan Liu <net147@gmail.com>