rs6000, add overloaded DFP quantize support

Add decimal floating point (DFP) quantize built-ins for both 64-bit DFP
and 128-DFP operands.  In each case, there is an immediate version and a
variable version of the built-in.  The RM value is a 2-bit constant int
which specifies the rounding mode to use.  For the immediate versions of
the built-in, the TE field is a 5-bit constant that specifies the value of
the ideal exponent for the result.  The built-in specifications are:

  __Decimal64 builtin_dfp_quantize (_Decimal64, _Decimal64,
				    const int RM)
  __Decimal64 builtin_dfp_quantize (const int TE, _Decimal64,
				    const int RM)
  __Decimal128 builtin_dfp_quantize (_Decimal128, _Decimal128,
				     const int RM)
  __Decimal128 builtin_dfp_quantize (const int TE, _Decimal128,
				     const int RM)

A testcase is added for the new built-in definitions.

gcc/ChangeLog:
	* config/rs6000/dfp.md (UNSPEC_DQUAN): New unspec.
	(dfp_dqua_<mode>, dfp_dquai_<mode>): New define_insn.
	* config/rs6000/rs6000-builtins.def (__builtin_dfp_dqua,
	__builtin_dfp_dquai, __builtin_dfp_dquaq, __builtin_dfp_dquaqi):
	New buit-in definitions.
	* config/rs6000/rs6000-overload.def (__builtin_dfp_quantize): New
	overloaded definition.
	* doc/extend.texi: Add documentation for __builtin_dfp_quantize.

gcc/testsuite/
	* gcc.target/powerpc/pr93448.c: New test case.

	PR target/93448

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