qemu: fix do_compile error

Fix error like:
Fatal error: can't create tests/ptimer-test.p/..._qemu-5.2.0_hw_core_ptimer.c.o: File name too long

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>