[build] Fix __libgcc attribute for recent gcc versions

We observed some time ago (in commit 4ce8d61 "Import various libgcc
functions from syslinux") that gcc seems to treat calls to the
implicit arithmetic functions (e.g. __udivdi3()) as being affected by
-mregparm but unaffected by -mrtd.

This seems to be no longer the case with current gcc versions, which
treat calls to these functions as being affected by both -mregparm and
-mrtd, as expected.

There is nothing obvious in the gcc changelogs to indicate precisely
when this happened.  From experimentation with available gcc versions,
the change occurred sometime between v4.6.3 and v4.7.2.  We assume
that only versions up to v4.6.x require the special treatment.

Signed-off-by: Michael Brown <mcb30@ipxe.org>