[gdb/testsuite] Add cc-with-dwz.exp and cc-with-dwz-m.exp

[gdb/testsuite] Add cc-with-dwz.exp and cc-with-dwz-m.exp

We can use CC_WITH_TWEAKS_FLAGS when cd-ing into the gdb build subdir and
invoking make check:
...
$ cd $objdir/gdb
$ make check \
    RUNTESTFLAGS='--target_board=cc-with-tweaks' \
    CC_WITH_TWEAKS_FLAGS='-z'
...

But when cd-ing into the top-level build dir and invoking make check-gdb
instead:
...
$ cd $objdir
$ make check-gdb \
    RUNTESTFLAGS='--target_board=cc-with-tweaks' \
    CC_WITH_TWEAKS_FLAGS='-z'
...
using CC_WITH_TWEAKS_FLAGS has no effect, because CC_WITH_TWEAKS_FLAGS is not
passed down from the top level Makefile.

Add cc-with-dwz.exp and cc-with-dwz-m.exp, that don't require
CC_WITH_TWEAKS_FLAGS to be set in the make invocation, allowing us to run these
test configurations from the toplevel build dir:
...
$ cd $objdir
$ make check-gdb \
    RUNTESTFLAGS='--target_board=cc-with-dwz'
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-04-11  Tom de Vries  <tdevries@suse.de>

	* boards/cc-with-dwz-m.exp: New file.
	* boards/cc-with-dwz.exp: New file.
	* boards/cc-with-tweaks.exp: Note that check-gdb doesn't work.