treewide: add support for "gc-sections" in PKG_BUILD_FLAGS

treewide: add support for "gc-sections" in PKG_BUILD_FLAGS

This reduces open coding and allows to easily add a knob to
enable it treewide, where chosen packages can still opt-out via
"no-gc-sections".

Note: libnl, mbedtls and opkg only used the CFLAGS part without the
LDFLAGS counterpart. That doesn't help at all if the goal is to produce
smaller binaries. I consider that an accident, and this fixes it.

Note: there are also packages using only the LDFLAGS part. I didn't
touch those, as gc might have been disabled via CFLAGS intentionally.

Signed-off-by: Andre Heider <a.heider@gmail.com>