[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.