testsuite: Extend TLS core file testing with an OS-generated dump

testsuite: Extend TLS core file testing with an OS-generated dump

Complementing commit 280ca31f4d60 ("Add test for fetching TLS from
core file") extend gdb.threads/tls-core.exp with an OS-generated dump
where supported.

This verifies not only that our core dump interpreter is consistent
with our producer, but that it matches the OS verified as well,
avoiding a possible case where our interpreter would be bug-compatible
with our producer but not the OS and it would go unnoticed in testing.

This results in:

 PASS: gdb.threads/tls-core.exp: native: load core file
 PASS: gdb.threads/tls-core.exp: native: print thread-local storage variable
 PASS: gdb.threads/tls-core.exp: gcore: load core file
 PASS: gdb.threads/tls-core.exp: gcore: print thread-local storage variable

with local testing and:

 UNSUPPORTED: gdb.threads/tls-core.exp: native: load core file
 UNSUPPORTED: gdb.threads/tls-core.exp: native: print thread-local storage variable
 PASS: gdb.threads/tls-core.exp: gcore: load core file
 PASS: gdb.threads/tls-core.exp: gcore: print thread-local storage variable

with remote testing, or for testing on ports that don't supports
cores.

gdb/testsuite/ChangeLog:
2018-05-24  Maciej W. Rozycki  <macro@mips.com>
	    Pedro Alves  <palves@redhat.com>

	* gdb.threads/tls-core.c: Include <stdlib.h>
	(thread_proc): Call `abort'.
	* gdb.threads/tls-core.exp: Generate a core with core_find too.
	(tls_core_test): New procedure, bits factored out from ...
	(top level): ... here.  Test both native cores and gcore cores.