gdb.python/py-unwind: Disable stack protection

gdb.python/py-unwind: Disable stack protection

[I made some typo fixes but forgot to amend my commit before sending the patch,
 hence this v2.]

I see the following failure on Ubuntu 16.04's gcc 5.4.0:

Running /home/emaisin/src/binutils-gdb/gdb/testsuite/gdb.python/py-unwind.exp ...
FAIL: gdb.python/py-unwind.exp: continue to breakpoint: break backtrace-broken
FAIL: gdb.python/py-unwind.exp: Backtrace restored by unwinder (pattern 1)

The problem is that the test expects a very particular stack layout.
When stack protection is enabled, it adds a canary value which looks
like an additional local variable.  This makes the test complain about
a bad stack layout and fail.

The simple solution is to disable stack protection for that test using
-fno-stack-protector.  I checked older compilers (gcc 4.4, clang 3.5)
and they support that flag, so I don't think it's necessary to probe for
whether the compiler supports it.

Maybe a better solution would be to change the test to make it cope with
different stack layouts (perhaps it could save addresses of stuff in
some global variables which GDB/the unwinder would read).  I'll go with
the simple solution for now though.

gdb/testsuite/ChangeLog:

	* gdb.python/py-unwind.exp: Disable stack protection when
	building test file.