ipxe: use gcc6 for more compact code

We have a strict size limit of 64K which needs to be enforced for
pxe-virtio.rom for migration compatibility. The v4.8 gcc compiler
used in SLE12 doesn't produce as compact code as does more recent
gcc compilers, and the pxe-virtio.rom produced with it doesn't fit
in 64K anymore without taking extraordinary efforts. I was unable
to find enough opportunities in the ipxe code and data to reduce
its size, and found that using a more recent compiler would be the
best solution to keeping the rom size within the 64K size limit.

Signed-Off-By: Bruce Rogers <brogers@suse.com>
--- a/src/Makefile
+++ b/src/Makefile
@@ -24,7 +24,7 @@ ECHO		:= echo
 PRINTF		:= printf
 PERL		:= perl
 TRUE		:= true
-CC		:= $(CROSS_COMPILE)gcc
+CC		:= $(CROSS_COMPILE)gcc-6
 CPP		:= $(CC) -E
 AS		:= $(CROSS_COMPILE)as
 LD		:= $(CROSS_COMPILE)ld