toolchain/gcc: pass proper flags for arm hard float build

This fixes eglibc build failure. More information about the bug can be found on
the link below.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61796

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41657 3c298f89-4303-0410-b956-a3cf2f4a3e73