UPSTREAM: xcompile: add riscv64 name to riscv toolchain

In newer toolchain with binutils 2.26 and GCC 5.3.0, we build binutils
and GCC with machine type riscv32 and riscv64 instead of riscv. We can
see it in this riscv-gnu-toolchain commit:
dedbf07

Signed-off-by: Iru Cai <mytbk920423@gmail.com>
Signed-off-by: Martin Roth <martinroth@google.com>
BUG=None
BRANCH=None
TEST=None

Reviewed-on: https://review.coreboot.org/14257
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Change-Id: Id552859ec256d80108e073d25cd51dd1fc3fbfac
Reviewed-on: https://chromium-review.googlesource.com/371505
Commit-Ready: Furquan Shaikh <furquan@chromium.org>
Tested-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-by: Martin Roth <martinroth@chromium.org>

util/xcompile/xcompile

@@ -332,7 +332,7 @@ arch_config_arm64() {
 arch_config_riscv() {
 	TARCH="riscv"
 	TBFDARCHS="littleriscv"
-	TCLIST="riscv"
+	TCLIST="riscv riscv64"
 	TWIDTH="64"
 	TABI="elf"
 }