libgfortran: Fix bootstrap on targets without static_assert macro.

Although we build the library with GCC which is known to support
_Static_assert this might be done on a system without the macro
mapping static_assert to the compiler keyword.

The use of static_assert introduced with r12-6126-g3430132f3e82
causes bootstrap to fail on such targets, fixed by using the keyword
directly.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

libgfortran/ChangeLog:

	* runtime/string.c (gfc_itoa): Use _Static_assert directly
	instead of via the static_assert macro.