Fix Mac OS X build, debugserver version handling.

Summary:
No longer rely on cmake to set DEBUGSERVER_VERSION_STR,
but now generate the _vers.c file like xcode does
and include the generated file into the build on Mac OS X.

This fixes the cmake Mac OS X build after an earlier change
by Jason Molenda.

Reviewers: clayborg, jasonmolenda

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11450

llvm-svn: 243072