Enable TestReadMemCString on non-darwin targets

The test works fine on linux, and I believe other targets should not
have an issue with as well. If they do, we can start blacklisting
instead of whitelisting.

The idea of using "-1" as the value of the pointer on non-apple targets
backfired, as it fails the "address != LLDB_INVALID_ADDRESS" test (-1 is
the value of LLDB_INVALID_ADDRESS).  However, it should be safe to use
0x100 for other targets as well. The first page of memory is generally
kept unreadable to catch null pointer dereferences.

llvm-svn: 321353