[RISC-V][PR target/118170] Add HF div/sqrt reservation

Clearly an oversight in the generic-ooo model caught by the checking code.  I
should have realized it was generic-ooo as we don't have a pipeline description
for the tenstorrent design yet, just the costing model.

The patch was extracted from the BZ which indicated Anton was the author, so I
kept that.  I'm listed as co-author just in case someone wants to complain
about the testcase in the future.  I didn't do any notable lifting here.

Thanks Peter and Anton!

	PR target/118170
gcc/
	* config/riscv/generic-ooo.md (generic_ooo_float_div_half): New
	reservation.

gcc/testsuite
	* gcc.target/riscv/pr118170.c: New test.

Co-authored-by: Jeff Law <jlaw@ventanamicro.com>