Enable the SafeStack sanitizer on CloudABI by default.

Over the last month we've been testing SafeStack extensively. As far as
we know, it works perfectly fine. That why I'd like to see us having
this enabled by default for CloudABI.

This change introduces a getDefaultSanitizers() function that toolchains
can use to specify which sanitizers are enabled by default. Once all
flags are processed, only flags that had no -fno-sanitize overrides are
enabled.

Extend the thests for CloudABI to test both the default case and the
case in which we want to explicitly disable SafeStack.

Reviewed by:	eugenis, pcc
Differential Revision:	http://reviews.llvm.org/D18505

llvm-svn: 264787