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.

ChangeLog

@@ -1,5 +1,10 @@
 2015-02-26  Joseph Myers  <joseph@codesourcery.com>
 
+	* 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.
+
 	* sysdeps/ieee754/dbl-64/e_j0.c (pzero): Change last case for
 	setting p and q from "else if" to "else".
 	(qzero): Likewise.

sysdeps/ieee754/Makefile

@@ -1,5 +1,4 @@
 ifeq ($(subdir),math)
 sysdep_headers += ieee754.h
-CFLAGS-k_standard.c = -Wno-write-strings
 endif