buildgcc: Downgrade to gcc 4.7.3, handle armv7-a

gcc 4.8.x has issues with using ebp, which broke some builds,
so downgrade. The problem also manifested elsewhere, so it's
not necessarily our fault.

While at it, gcc complained about "armv7a" where it seems to
expect "armv7-a".

Change-Id: I6f0c35f49709cb41022475bb47116c12ab1c7ee3
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/3930
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

util/xcompile/xcompile

@@ -140,7 +140,7 @@ SUPPORTED_ARCHITECTURE="x86 armv7"
 # ARM Architecture
 TARCH_armv7="armv7"
 TBFDARCH_armv7="littlearm"
-TCLIST_armv7="armv7a"
+TCLIST_armv7="armv7a armv7-a"
 TWIDTH_armv7="32"
 
 # X86 Architecture