powerpc: change .fixup test to a GCC version test

This was introduced by commit 24461519, but it
fails in a minimal SPL build where the only thing in arch/powerpc/lib is
cache.c, which apparently doesn't generate any fixup records.

The problem is reported to occur with GCC 3.x, so insist on GCC 4.0 or newer.
Patterned after checkthumb as suggested by Tom Rini.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Peter Tyser <ptyser@xes-inc.com>
Cc: Tom Rini <trini@ti.com>
--
v2: test gcc version instead of testing nothing