[testsuite, nvptx] Fix gcc.dg/tls/thr-cse-1.c

[testsuite, nvptx] Fix gcc.dg/tls/thr-cse-1.c

With nvptx, we run into:
...
FAIL: gcc.dg/tls/thr-cse-1.c scan-assembler-not \
  emutls_get_address.*emutls_get_address.*
...
because the nvptx assembly looks like:
...
  call (%value_in), __emutls_get_address, (%out_arg1);
  ...
// BEGIN GLOBAL FUNCTION DECL: __emutls_get_address
.extern .func (.param.u64 %value_out) __emutls_get_address (.param.u64 %in_ar0);
...

Fix this by checking the slim final dump instead, where we have just:
...
   12: r35:DI=call [`__emutls_get_address'] argc:0
...

gcc/testsuite/ChangeLog:

2020-09-24  Tom de Vries  <tdevries@suse.de>

	* gcc.dg/tls/thr-cse-1.c: Scan final dump instead of assembly for
	nvptx.