Support 'make check-parallel' in gdb's build dir

Currently, you can cd to the gdb/testsuite/ dir and use
make check-parallel, instead of using FORCE_PARALLEL:

 $ make -j8 check-parallel RUNTESTFLAGS="--target_board=native-gdbserver"
 $ make -j8 check RUNTESTFLAGS="--target_board=native-gdbserver" FORCE_PARALLEL=1

But you can't do that in the build/gdb/ dir:

 $ make check-parallel RUNTESTFLAGS="--target_board=native-gdbserver"
 make: *** No rule to make target `check-parallel'.  Stop.

I find check-parallel a bit more convenient, and more typo-proof, so
this patch makes it work from the gdb build dir too.

While documenting this in testsuite/README, I found that the parallel
testing mode would better be pulled out to its own section and
extended.

gdb/ChangeLog:
2016-02-11  Pedro Alves  <palves@redhat.com>

	* Makefile.in (check-parallel): New rule.

gdb/testsuite/ChangeLog:
2016-02-11  Pedro Alves  <palves@redhat.com>

	* README (Parallel testing): New section.
	(GDB_PARALLEL): Rewrite.
	(FORCE_PARALLEL): Document.

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