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>

util/xcompile/xcompile

@@ -184,7 +184,7 @@ arch_config_aarch64() {
 	TBFDARCH="littleaarch64"
 	TCLIST="aarch64"
 	TWIDTH="64"
-	TABI="eabi"
+	TABI="elf"
 }
 
 arch_config_riscv() {