gcc: define USE_PT_GNU_EH_FRAME for musl as well, reduces binary object size

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40874 3c298f89-4303-0410-b956-a3cf2f4a3e73