[gn build] Process .def.in files in llvm/Config and add lib/Target/BUILD.gn

Tweak write_cmake_config.py to also handle variable references looking @FOO@
(matching CMake's configure_file() function), and make it replace '\' 'n' in
values with a newline literal since there's no good portable way of passing a
real newline literal on a command line.

Use that to process all the .def.in files in llvm/include/Config and add
llvm/lib/Target/BUILD.gn, which (indirectly, through llvm-c/Target.h) includes
them.

Differential Revision: https://reviews.llvm.org/D55184

llvm-svn: 348503