Require GLIBC 2.32 for Decimal/_Float128 conversions.

Require GLIBC 2.32 for Decimal/_Float128 conversions.

In the patch that I applied on March 2nd, I had code to provide support for
Decimal/_Float128 conversions if the user did not use at least GLIBC 2.32.  It
did this by using __ibm128 as an intermediate type.  The trouble is __ibm128
cannot represent all of the numbers that _Float128 can, and you lose if you do
this conversion.

This patch removes this support.  The dfp-bit.c functions now call the the
__sprintfieee128 and __strtoieee128 functions to do the conversion.  If the
user does not have GLIBC, they will get a linker error that these functions do
not exist.

The float128 support functions are only built into the static libgcc, so there
isn't an issue with having references to __strtoieee128 and __sprintfieee128
with older GLIBC libraries.

As an added bonus, this patch eliminates the __sprintfkf function which
included stdio.h to get a definition for the sprintf library function.  This
allows for building cross compilers without having to have a target stdio.h
available.

libgcc/
2021-03-29  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/t-float128 (fp128_decstr_funcs): Delete.
	(fp128_ppc_funcs): Do not add $(fp128_decstr_funcs).
	(fp128_decstr_objs): Delete.
	* dfp-bit.h: Call __sprintfieee128 to do conversions from
	_Float128 to a Decimal type.  Call __strtoieee128 to do
	conversions from a Decimal type to _Float128.
	* config/rs6000/_sprintfkf.c: Delete file.
	* config/rs6000/_sprintfkf.h: Delete file.
	* config/rs6000/_strtokf.c: Delete file.
	* config/rs6000/_strtokf.h: Delete file.