x86: Harden printf against non-normal long double values (bug 26649)

The behavior of isnan/__builtin_isnan on bit patterns that do not
correspond to something that the CPU would produce from valid inputs
is currently under-defined in the toolchain. (The GCC built-in and
glibc disagree.)

The isnan check in PRINTF_FP_FETCH in stdio-common/printf_fp.c
assumes the GCC behavior that returns true for non-normal numbers
which are not specified as NaN. (The glibc implementation returns
false for such numbers.)

At present, passing non-normal numbers to __mpn_extract_long_double
causes this function to produce irregularly shaped multi-precision
integers, triggering undefined behavior in __printf_fp_l.

With GCC 10 and glibc 2.32, this behavior is not visible because
__builtin_isnan is used, which avoids calling
__mpn_extract_long_double in this case.  This commit updates the
implementation of __mpn_extract_long_double so that regularly shaped
multi-precision integers are produced in this case, avoiding
undefined behavior in __printf_fp_l.