Fix i386 fdim double rounding (bug 20255).

Fix i386 fdim double rounding (bug 20255).

fdim suffers from double rounding on i386 because subtracting two
double values can produce an inexact long double value exactly half
way between two double values.  This patch fixes this by creating an
i386-specific version of fdim - C, based on the generic version,
unlike the previous .S version - which sets the x87 precision control
to double precision for the subtraction and then restores it
afterwards.  As noted in the comment added, there are no issues of
double rounding for subnormals (a case that setting precision control
does not address) because subtraction cannot produce an inexact result
in the subnormal range.

Tested for x86_64 and x86.

	[BZ #20255]
	* sysdeps/i386/fpu/s_fdim.c: New file.  Based on math/s_fdim.c.
	* math/libm-test.inc (fdim_test_data): Add another test.