Fix riscv build, no longer works with python2

Building riscv no longer works with python2:

> python ./config/riscv/arch-canonicalize -misa-spec=20191213 rv64gc
  File "./config/riscv/arch-canonicalize", line 229
    print(f"ERROR: Unhandled conditional dependency: '{ext_name}' with condition:", file=sys.stderr)
                                                                                 ^
SyntaxError: invalid syntax

On systems that have python aliased to python2 we chose that, even
when python3 is available.  Don't.

	* config.gcc (riscv*-*-*): Look for python3, then fall back
	to python.  Never use python2.

gcc/config.gcc

@@ -4782,7 +4782,7 @@ case "${target}" in
 			exit 1
 			;;
 		esac
-		PYTHON=`which python || which python3 || which python2`
+		PYTHON=`which python3 || which python`
 		if test "x${PYTHON}" != x; then
 			with_arch=`${PYTHON} ${srcdir}/config/riscv/arch-canonicalize -misa-spec=${with_isa_spec} ${with_arch}`
 		fi