Retrieve command line arguments and environment correctly on FreeBSD

Summary:
Recently I saw the test `TestCases/Posix/print_cmdline.cc` failing on
FreeBSD, with "expected string not found in input".  This is because
asan could not retrieve the command line arguments properly.

In `lib/sanitizer_common/sanitizer_linux.cc`, this is taken care of by
the `GetArgsAndEnv()` function, but it uses `__libc_stack_end` to get at
the required data.  This variable does not exist on BSDs; the regular
way to retrieve the arguments and environment information is via the
`kern.ps_strings` sysctl.

I added this functionality in sanitizer_linux.cc, as a separate #ifdef
block in `GetArgsAndEnv()`.  Also, `ReadNullSepFileToArray()` becomes
unused due to this change.  (It won't work on FreeBSD anyway, since
`/proc` is not mounted by default.)

Reviewers: kcc, emaste, joerg, davide

Subscribers: llvm-commits, emaste

Differential Revision: http://reviews.llvm.org/D17832

llvm-svn: 263157