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>
Change-Id: Id552859ec256d80108e073d25cd51dd1fc3fbfac
Reviewed-on: https://review.coreboot.org/14257
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

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"
 }