[spu] Some additional test fixes

[spu] Some additional test fixes

Now that the ppc64 breakpoint regression is fixed, running the
gdb.cell test suite showed a few more test case problems, caused
by tests that haven't been updated to adapt to GDB changes.

gdb/testsuite/ChangeLog:
2017-11-30  Ulrich Weigand  <uweigand@de.ibm.com>

	* gdb.cell/gcore.exp: Fix typo when setting spu_bin.
	Update for changed thread numbering.
	* gdb.cell/bt.exp: Update for changed GDB output.