crossgcc: Add x86_64 to list of supported architectures

You can build your new toolchain with:
  $ cd util/crossgcc/
  $ ./buildgcc -d /opt/cross -p x86_64-elf -j 16
or
  $ make crossgcc-x64

Change-Id: I8eb584166294578d2b33c63e94ed3aca9b5de4f4
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/8668
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/Makefile.inc b/Makefile.inc
index 656d988..d4748fc 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -424,12 +424,15 @@
 	git config remote.origin.push HEAD:refs/for/master
 	(git config --global user.name >/dev/null && git config --global user.email >/dev/null) || (printf 'Please configure your name and email in git:\n\n git config --global user.name "Your Name Comes Here"\n git config --global user.email your.email@example.com\n'; exit 1)
 
-crossgcc: crossgcc-i386 crossgcc-arm crossgcc-aarch64 crossgcc-mips crossgcc-riscv
+crossgcc: crossgcc-i386 crossgcc-x64 crossgcc-arm crossgcc-aarch64 crossgcc-mips crossgcc-riscv
 
-.PHONY: crossgcc-i386 crossgcc-arm crossgcc-aarch64 crossgcc-mips crossgcc-riscv
+.PHONY: crossgcc-i386 crossgcc-x64 crossgcc-arm crossgcc-aarch64 crossgcc-mips crossgcc-riscv
 crossgcc-i386: clean-for-update
 	$(MAKE) -C util/crossgcc build-i386-without-gdb
 
+crossgcc-x64: clean-for-update
+	$(MAKE) -C util/crossgcc build-x64-without-gdb
+
 crossgcc-arm: clean-for-update
 	$(MAKE) -C util/crossgcc build-armv7a-without-gdb
 
@@ -442,12 +445,15 @@
 crossgcc-riscv: clean-for-update
 	$(MAKE) -C util/crossgcc build-riscv-without-gdb
 
-crosstools: crosstools-i386 crosstools-arm crosstools-aarch64 crosstools-mips crosstools-riscv
+crosstools: crosstools-i386 crosstools-x64 crosstools-arm crosstools-aarch64 crosstools-mips crosstools-riscv
 
-.PHONY: crosstools-i386 crosstools-arm crosstools-aarch64 crosstools-mips crosstools-riscv
+.PHONY: crosstools-i386 crosstools-x64 crosstools-arm crosstools-aarch64 crosstools-mips crosstools-riscv
 crosstools-i386: clean-for-update
 	$(MAKE) -C util/crossgcc build-i386
 
+crosstools-x64: clean-for-update
+	$(MAKE) -C util/crossgcc build-x64
+
 crosstools-arm: clean-for-update
 	$(MAKE) -C util/crossgcc build-armv7a