Default to IMAFD, not G for --with-arch

G doesn't actually work.

configure.ac

@@ -57,10 +57,10 @@ AC_ARG_WITH(xlen,
 	)
 
 AC_ARG_WITH(arch,
-	[AS_HELP_STRING([--with-arch=G],
-		[Sets the base RISC-V ISA, defaults to G])],
+	[AS_HELP_STRING([--with-arch=IMAFD],
+		[Sets the base RISC-V ISA, defaults to IMAFD])],
 	AC_SUBST(ARCH, $withval),
-	AC_SUBST(ARCH, G)
+	AC_SUBST(ARCH, IMAFD)
 	)
 
 AC_ARG_ENABLE(atomic,