[gdb/testsuite] Fix stray file in get_compiler_info

When running test-case gdb.dwarf2/gdb-index-nodebug.exp with host board
local-remote-host and target board remote-gdbserver-on-localhost, I get:
...
$ ls build/gdb/testsuite
cache    compiler.i  config.log  config.status  gdb.log  gdb.sum  lib  Makefile
outputs  site.bak    site.exp    temp
...

The file compiler.i is there because get_compiler_info uses:
...
	set ppout "$outdir/compiler.i"
...

The file is a temporary, and as such belongs in a temp dir.  Fix this by using
standard_temp_file, moving the file to build/gdb/testsuite/temp/<pid>/compiler.i.

Tested on x86_64-linux.