Un-conditionalize use of libcompression. debugserver only builds

Un-conditionalize use of libcompression.  debugserver only builds
on Darwin systems and libcompression has been in the OS for over
three years.

Remove use of / linking to zlib.  We'll always have libcompression
available now.

Create a scratch buffer via compression_encode_scratch_buffer_size()
and use it in calls to compression_encode_buffer() to avoid
compression_encode_buffer having to malloc & free a scratch buffer
on each call.

Tested by forcing compression to be enabled on macos native (normally
only enabled on iOS et al devices), running the testsuite.

<rdar://problem/41601084>

llvm-svn: 349553