rs6000: Add Rust support to traceback table

rs6000: Add Rust support to traceback table

Use 0 for the "lang" identifier for Rust, just like we do for all other
source languages without assigned language code (0 means "C").

Tested on powerpc64-linux.  Without this patch there are ICEs galore in
the gm2 testsuite for 64-bit Linux targets, and with the ptch there are
just a few FAILs.

2022-12-17  Segher Boessenkool  <segher@kernel.crashing.org>

	* config/rs6000/rs6000-logue.cc (rs6000_output_function_epilogue):
	Handle GNU Rust for the tbtab lang field.