gcc: do not delete DATESTAMP and DEV-PHASE version info (fixes __GLIBCXX__ definition)

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

git-svn-id: svn://svn.openwrt.org/openwrt@37179 3c298f89-4303-0410-b956-a3cf2f4a3e73