[RISC-V][PR target/109279] Improve RISC-V constant synthesis

This is a small improvement to the constant synthesis code to capture a case
appended to PR 109279.

The case in question has the property that the high 32 bits have the value one
less than the low 32 bits and the highest bit in two low 32 bits is on.  The
example used in BZ is 0xcccccccccccccccd which comes up computing N/10.

When we construct a constant with bit 31 on, it gets implicitly sign extended.
So something like 0xcccccccd when constructed would generate
0xffffffffcccccccd.  The low bits are precisely what we want and the high bits
are a "-1".  Both properties are useful.

We left shift that value by 32 positions into a temporary and add that
temporary to the original value.  Concretely:

  0xffffffffcccccccd
+ 0xcccccccd00000000
  ------------------
  0xcccccccccccccccd

Tested in my tester on rv32 and rv64, waiting on the pre-commit tester to do its thing.

	PR target/109279
gcc/
	* config/riscv/riscv.cc (riscv_build_integer): Handle another 64-bit
	synthesis where high half is one less than the low half and the 32-bit
	sign bit is on.

gcc/testsuite/

	* gcc.target/riscv/synthesis-16.c: New test.

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