Use type-specific precision when printing results in libm-test.inc.

When libm-test.inc prints the results of failing tests, the output can
be unhelpful for ldbl-128 and ldbl-128ibm because the precision used
is insufficient to distinguish values of those types, resulting in
reported values that look identical but differ by a large number of
ulps.

This patch changes it to use a precision appropriate for the type, for
both decimal and hex output (so output for float is more compact,
output for ldbl-128 and ldbl-128ibm is substantially wider).  The
natural precision to use for decimal is given by the C11 <float.h>
macros such as FLT_DECIMAL_DIG.  GCC's <float.h> only defines those in
C11 mode, so this patch uses the predefines such as
__FLT_DECIMAL_DIG__ (added in GCC 4.6) instead; if we move to building
with -std=gnu11 (or -std=gnu1x if we can't get rid of 4.6 support).

Tested for powerpc and mips64.

	* math/libm-test.inc (TYPE_DECIMAL_DIG): New macro.
	(TYPE_HEX_DIG): Likewise.
	(print_float): Use TYPE_DECIMAL_DIG - 1 and TYPE_HEX_DIG - 1 as
	precisions when printing floating-point numbers.
	(check_float_internal): Likewise.