gdb.trace: Add a testcase for tdesc in tfile.

gdb.trace: Add a testcase for tdesc in tfile.

This tests whether $ymm15 can be correctly collected and printed from
tfile.  It covers:

- storing tdesc in tfile (without that, $ymm15 doesn't exist)
- ax_pseudo_register_collect for x86 (without that, $ymm15 cannot be
  collected)
- register order in tfile_fetch_registers (without that, $ymm15h is
  fetched from wrong position)
- off-by-one in tfile_fetch_registers (without that, $ymm15h is
  incorrectly considered to be out of bounds)
- using proper tdesc in encoding tracepoint actions (without that,
  internal error happens due to $ymm15h being considered unavailable)

gdb/testsuite/ChangeLog:

	* gdb.trace/tfile-avx.c: New test.
	* gdb.trace/tfile-avx.exp: New test.