Fix scan-assembler regexp in recurse2.rs

gcc/testsuite/ChangeLog:

	* rust/compile/macros/builtin/recurse2.rs: Match "abheyho\0" as
	well as "abheyho", to handle slight differences in assembly
	output for null-terminated strings.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>

Author: powerboat9

gcc/testsuite/rust/compile/macros/builtin/recurse2.rs

@@ -34,7 +34,7 @@ fn print_str(s: &str) {
     }
 }
 
-// { dg-final { scan-assembler {"abheyho"} } }
+// { dg-final { scan-assembler {"abheyho(\\0)?"} } }
 static S: &str = concat!("a", 'b', a!(), a!(b c d e f a!()), '\0');
 
 fn main() {