package/gcc: fix libgcc build for xtensa

xtensa libgcc can't be built with -mtext-section-literals flag, now
coming from TARGET_CFLAGS, because it needs to emit literals to
.init/.fini sections, which is not currently supported.

Filter -mtext-section-literals flag out of GCC_COMMON_TARGET_CFLAGS.

Suggested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>