Fix libm feholdexcept namespace (bug 17748).

Fix libm feholdexcept namespace (bug 17748).

Continuing the fixes for C90 libm functions calling C99 fe* functions,
this patch fixes the case of feholdexcept by making it a weak alias of
__feholdexcept and making the affected code call __feholdexcept.

Tested for x86_64 (testsuite, and that disassembly of installed shared
libraries is unchanged by the patch).  Also tested for ARM
(soft-float) that feholdexcept failures disappear from the
linknamespace test failures (fesetenv, fsetround and feupdateenv
remain to be addressed to complete fixing bug 17748).

	[BZ #17748]
	* include/fenv.h (__feholdexcept): Declare.  Use
	libm_hidden_proto.
	* math/feholdexcpt.c (feholdexcept): Rename to __feholdexcept and
	define as weak alias of __feholdexcept.  Use libm_hidden_weak.
	* sysdeps/aarch64/fpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/alpha/fpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/arm/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/hppa/fpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/i386/fpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/ia64/fpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/m68k/fpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/mips/fpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/powerpc/fpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/powerpc/nofpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/powerpc/powerpc32/e500/nofpu/feholdexcpt.c
	(feholdexcept): Likewise.
	* sysdeps/s390/fpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/sh/sh4/fpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/sparc/fpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/x86_64/fpu/feholdexcpt.c (feholdexcept): Likewise.
	* sysdeps/generic/math_private.h (default_libc_feholdexcept): Use
	__feholdexcept instead of feholdexcept.
	(default_libc_feholdexcept_setround): Likewise.