c-family: Support DFP printf/scanf formats for C2X

c-family: Support DFP printf/scanf formats for C2X

When I enabled various decimal floating-point features for C2X /
stopped them being diagnosed with -pedantic for C2X, I missed the
format checking support.  The DFP printf and scanf formats are
included in C2X.  Thus, adjust the data for those formats so that they
are no longer diagnosed with -std=c2x -Wformat -pedantic.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/c-family/
	* c-format.c (printf_length_specs, scanf_length_specs)
	(print_char_table, scan_char_table): Support DFP formats for C2X.
	* c-format.h (TEX_D32, TEX_D64, TEX_D128): Remove.
	(T2X_D32, T2X_D64, T2X_D128): New macros.

gcc/testsuite/
	* gcc.dg/format/c11-dfp-printf-1.c,
	gcc.dg/format/c11-dfp-scanf-1.c, gcc.dg/format/c2x-dfp-printf-1.c,
	gcc.dg/format/c2x-dfp-scanf-1.c: New tests.