Simplify x86 nearbyint functions.

The i386 implementations of nearbyint functions, and x86_64
nearbyintl, contain code to mask the "inexact" exception.  However,
the fnstenv instruction has the effect of masking all exceptions, so
this masking code has been redundant since fnstenv was added to those
implementations (by commit 846d9a4a;
commit 71d1b016 added the test
math/test-nearbyint-except-2.c that verifies these functions do work
when called with "inexact" traps enabled); this patch removes the
redundant code.

Tested for x86_64 and x86.

	* sysdeps/i386/fpu/s_nearbyint.S (__nearbyint): Do not mask
	"inexact" exceptions after fnstenv.
	* sysdeps/i386/fpu/s_nearbyintf.S (__nearbyintf): Likewise.
	* sysdeps/i386/fpu/s_nearbyintl.S (__nearbyintl): Likewise.
	* sysdeps/x86_64/fpu/s_nearbyintl.S (__nearbyintl): Likewise.