real: fix encoding of negative IEEE double/quad values [PR98216]

real: fix encoding of negative IEEE double/quad values [PR98216]

In encode_ieee_double/quad, the assignment

  unsigned long WORD = r->sign << 31;

is intended to set the 31st bit of WORD whenever the sign bit is set.
But on LP64 hosts it also unintentionally sets the upper 32 bits of WORD,
because r->sign gets promoted from unsigned:1 to int and then the result
of the shift (equal to INT_MIN) gets sign extended from int to long.

In the C++ frontend, this bug causes incorrect mangling of negative
floating point values because the output of real_to_target called from
write_real_cst unexpectedly has the upper 32 bits of this word set,
which the caller doesn't mask out.

This patch fixes this by avoiding the unwanted sign extension.  Note
that r0-53976 fixed the same bug in encode_ieee_single long ago.

	PR c++/98216
	PR c++/91292

gcc/ChangeLog:

	* real.c (encode_ieee_double): Avoid unwanted sign extension.
	(encode_ieee_quad): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-float2.C: New test.