libcore/libstd: fix commas in macro_rules! macros

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.