[libstdc++] [testsuite] require dfprt on some tests

[libstdc++] [testsuite] require dfprt on some tests

On a target that doesn't enable decimal float components in libgcc
(because the libc doens't define all required FE_* macros), but whose
compiler supports _Decimal* types, the effective target requirement
dfp passes, but several tests won't link because the runtime support
they depend on is missing.  State their dfprt requirement.

for  libstdc++-v3/ChangeLog

	* testsuite/decimal/binary-arith.cc: Require dfprt.
	* testsuite/decimal/comparison.cc: Likewise.
	* testsuite/decimal/compound-assignment.cc: Likewise.
	* testsuite/decimal/compound-assignment-memfunc.cc: Likewise.
	* testsuite/decimal/make-decimal.cc: Likewise.
	* testsuite/decimal/pr54036-1.cc: Likewise.
	* testsuite/decimal/pr54036-2.cc: Likewise.
	* testsuite/decimal/pr54036-3.cc: Likewise.
	* testsuite/decimal/unary-arith.cc: Likewise.