libcore/libstd: fix commas in macro_rules! macros

BREAKING CHANGE: (or perhaps, *bugfix*)

In #![no_std] applications, the following calls to `panic!` used
to behave differently; they now behave the same.

Old behavior:

    panic!("{{");   // panics with "{{"
    panic!("{{",);  // panics with "{"

New behavior:

    panic!("{{");   // panics with "{{"
    panic!("{{",);  // panics with "{{"

This only affects calls to `panic!` (and by proxy `assert`
and `debug_assert`) with a single string literal followed by
a trailing comma, and only in `#![no_std]` applications.

src/libstd/macros.rs

@@ -68,6 +68,9 @@ macro_rules! panic {
     ($msg:expr) => ({
         $crate::rt::begin_panic($msg, &(file!(), line!(), __rust_unstable_column!()))
     });
+    ($msg:expr,) => ({
+        panic!($msg)
+    });
     ($fmt:expr, $($arg:tt)+) => ({
         $crate::rt::begin_panic_fmt(&format_args!($fmt, $($arg)+),
                                     &(file!(), line!(), __rust_unstable_column!()))