Add test for fetching TLS from core file

Add test for fetching TLS from core file

A correct PID is needed by `libthread_db' library supplied with
glibc repository revisions before commit c579f48edba8 ("Remove
cached PID/TID in clone") or versions before 2.25 release for
GDB to fetch value of TLS variable from core file.  On MIPS
platforms it was omitted and fetching value of TLS variable was not
available.

This adds a new test in order to be sure if GDB on native platforms
can successfully fetch value of TLS variable.

gdb/testsuite:

	* gdb.threads/tls-core.c: New file.
	* gdb.threads/tls-core.exp: Likewise.