[gdb/testsuite] Fix gdb.threads/threadcrash.exp for remote host

[gdb/testsuite] Fix gdb.threads/threadcrash.exp for remote host

With test-case gdb.threads/threadcrash.exp using host board local-remote-host
and target board remote-gdbserver-on-localhost I run into:
...
(gdb) PASS: gdb.threads/threadcrash.exp: test_gcore: continue to crash
gcore $outputs/gdb.threads/threadcrash/threadcrash.gcore^M
Failed to open '$outputs/gdb.threads/threadcrash/threadcrash.gcore' for output.^M
(gdb) FAIL: gdb.threads/threadcrash.exp: test_gcore: saving gcore
UNSUPPORTED: gdb.threads/threadcrash.exp: test_gcore: couldn't generate gcore file
...

The problem is that the gcore command tries to save a file on a remote host,
but the filename is a location on build.

Fix this by using host_standard_output_file.

Tested on x86_64-linux.