[testsuite, nvptx] Fix string matching in gcc.dg/pr87314-1.c

[testsuite, nvptx] Fix string matching in gcc.dg/pr87314-1.c

with nvptx we run into:
...
FAIL: gcc.dg/pr87314-1.c scan-assembler hellooo
...

The required string is part of the assembly, just in a different format than
expected:
...
        .const .align 1 .u8 $LC0[12] =
	  { 104, 101, 108, 108, 111, 111, 111, 111, 98, 121, 101, 0 };
...

Fix this by adding an nvptx-specific scan-assembler directive.

Tested on nvptx and x86_64.

gcc/testsuite/ChangeLog:

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

	* gcc.dg/pr87314-1.c: Add nvptx-specific scan-assembler directive.