Use "qemu", not "riscv-qemu", as the default QEMU source

The recent addition of an override for the QEMU source directory left
the default pointing to the wrong submodule.

Fixes: b83ee52 ("Allow source-override for QEMU")
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

configure.ac

@@ -211,17 +211,17 @@ AC_DEFUN([AX_ARG_WITH_SRC],
 		)
 	  AS_IF([test "x$opt_name" != xdefault],
 		[AC_SUBST(opt_name,$opt_name)],
-		[AC_SUBST(opt_name,"\$(srcdir)/riscv-$1")])
+		[AC_SUBST(opt_name,"\$(srcdir)/$2")])
 	  m4_popdef([opt_name])
 	}])
 
-AX_ARG_WITH_SRC(gcc)
-AX_ARG_WITH_SRC(binutils)
-AX_ARG_WITH_SRC(newlib)
-AX_ARG_WITH_SRC(glibc)
-AX_ARG_WITH_SRC(musl)
-AX_ARG_WITH_SRC(gdb)
-AX_ARG_WITH_SRC(qemu)
+AX_ARG_WITH_SRC(gcc, riscv-gcc)
+AX_ARG_WITH_SRC(binutils, riscv-binutils)
+AX_ARG_WITH_SRC(newlib, riscv-newlib)
+AX_ARG_WITH_SRC(glibc, riscv-glibc)
+AX_ARG_WITH_SRC(musl, riscv-musl)
+AX_ARG_WITH_SRC(gdb, riscv-gdb)
+AX_ARG_WITH_SRC(qemu, qemu)
 
 AC_ARG_WITH(linux-headers-src,
 	[AC_HELP_STRING([--with-linux-headers-src],