[RISC-V] Fix ICE due to bogus use of gen_rtvec

[RISC-V] Fix ICE due to bogus use of gen_rtvec

Found this while setting up the risc-v coordination branch off of gcc-15.  Not
sure why I didn't use rtvec_alloc directly here since we're going to initialize
the whole vector ourselves.  Using gen_rtvec was just wrong as it's walking
down a non-existent varargs list.  Under the "right" circumstances it can walk
off a page and fault.

This was seen with a test already in the testsuite (I forget which test), so no
new regression test.

Tested in my tester and verified the failure on the coordination branch is
resolved a well.  Waiting on pre-commit CI to render a verdict.

gcc/
	* config/riscv/riscv-vect-permconst.cc (vector_permconst:process_bb):
	Use rtvec_alloc, not gen_rtvec since we don't want/need to initialize
	the vector.