Avoid -Wno-write-strings for k_standard.c.

We want to avoid -Wno- options in makefiles as far as possible, by
cleaning up the underlying issues if possible or failing that by using
diagnostic pragmas.  This patch eliminates the use of
-Wno-write-strings for sysdeps/ieee754/k_standard.c by using casts in
the source file to cast away const; those casts are encapsulated in a
macro that also deals with the choice of strings for float / double /
long double functions (for which the logic was previously replicated
many times).

Tested for x86_64; the only change to disassembly of installed
stripped shared libraries was a line number in an assertion.

	* sysdeps/ieee754/k_standard.c (CSTR): New macro.
	(__kernel_standard): Use CSTR macro when setting exc.name.
	* sysdeps/ieee754/Makefile [$(subdir) = math]
	(CFLAGS-k_standard.c): Remove variable.