RISC-V: Check multiletter extension has more than 1 letter

gcc/ChangeLog:

	* common/config/riscv/riscv-common.c
	(riscv_subset_list::parse_multiletter_ext): Checking multiletter
	extension has more than 1 letter.

gcc/testsuite/ChangeLog

	* gcc.target/riscv/arch-7.c: New.
	* gcc.target/riscv/attribute-10.c: Update test arch string.

gcc/common/config/riscv/riscv-common.c

@@ -524,6 +524,14 @@ riscv_subset_list::parse_multiletter_ext (const char *p,
 
       *q = '\0';
 
+      if (strlen (subset) == 1)
+	{
+	  error_at (m_loc, "%<-march=%s%>: name of %s must be more than 1 letter",
+		    m_arch, ext_type_str);
+	  free (subset);
+	  return NULL;
+	}
+
       add (subset, major_version, minor_version, explicit_version_p);
       free (subset);
       p += end_of_version - subset;

gcc/testsuite/gcc.target/riscv/arch-7.c

+/* { dg-do compile } */
+/* { dg-options "-O2 -march=rv32i -march=rv32im_s -mabi=ilp32" } */
+int foo()
+{
+}
+/* { dg-error ".'-march=rv32im_s': name of supervisor extension must be more than 1 letter" "" { target *-*-* } 0 } */

gcc/testsuite/gcc.target/riscv/attribute-10.c

 /* { dg-do compile } */
-/* { dg-options "-O2 -march=rv32i -march=rv32im_s_sx_unexpectedstring -mabi=ilp32" } */
+/* { dg-options "-O2 -march=rv32i -march=rv32im_sx_unexpectedstring -mabi=ilp32" } */
 int foo()
 {
 }