buildgcc: Add list of valid platform to the help text.

Change-Id: Ic48a08d1067c850555cf04ad29e65e9bdb7c4243
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: http://review.coreboot.org/10619
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marc.jones@se-eng.com>

util/crossgcc/buildgcc

@@ -305,6 +305,8 @@ myhelp()
 	printf "    [-p|--platform <platform>]    target platform to build cross compiler for\n"
 	printf "                                  (defaults to $TARGETARCH)\n"
 	printf "    [-S|--scripting]              build scripting support for GDB\n\n"
+	printf "Platforms for GCC & GDB:\n"
+	printf "    x86_64 i386-elf i386-mingw32 mipsel-elf riscv-elf arm aarch64\n\n"
 }
 
 myversion()