Mark fegetround pure (bug 16296).

Bug 16296 notes that fegetround is a pure function and should be
marked as such in fenv.h.  This patch implements that.

Tested for x86_64 and x86 (testsuite, and that installed stripped
shared libraries are unchanged by this patch).

	[BZ #16296]
	* math/fenv.h (fegetround): Use __attribute_pure__.
	* include/fenv.h (__fegetround): Likewise.