gcc: take into account ARM floating point capabilities

The ARM EABIhf support was introduced in gcc 4.6.x, so versions
earlier than this one should not be used when EABIhf is selected.

The ARM VFPv4 support was introduced in gcc 4.5.x, so versions earlier
than this one should not be used when VFPv4 is used.

[Peter: drop OABI dependencies]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>