crossgcc: Update GCC from 4.7.3 to 4.8.3 and update to version 1.25

gcc 4.9.2 fails on our tree right now. We should clean that
up and test before we make it the reference version.

Also, the AMD K8/Fam10 issue we had last year, for which
Vladimir provided an "untested" fix (which is in,
commit a6c29fe6841ad5e03ddb35803943bed3bc83dfd2), isn't
reproducible: I boot-tested an unpublished AMD K8 board
with coreboot built with gcc 4.8.3.
(Disclaimer: since the old issue depended on compiler
decisions on register allocation, any change to code
or compiler could mix up things in semi-random ways.)

Change-Id: I8f1460a8da2c9e2d581482b22a4824b10b8987fa
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/7526
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
crossgcc: Add buildsystem support for aarch64 compiler

This adds the crosstools-aarch64 and crossgcc-aarch64
make rules to create a toolchain (with or without gdb)
for AArch64 targets.

Also adapt xcompile, since it's aarch64-elf.

Change-Id: I6fbe09d44ee8b8493d3cd8dbbba869b409e311f7
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/7527
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>