gcc: musl patches only available for gcc 4.7+

For the moment, the musl support is not in mainline gcc, so it
requires a few patches. We have integrated those patches only for gcc
4.7 and gcc 4.8 at the moment, so only allow those gcc versions when
the musl library is selected.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>