cmELF: Allow building without system ELF headers

Use ELF headers vendored in `Utilities/cmelf` to get the ELF constants
and types. Using the same ELF definition header for all compilation
targets allows removing some #ifdefs depending on the host OS since we
know all required ELF constants will always be present. To reduce the
size of this commit, the CMake_USE_ELF_PARSER definite will be removed
in a separate commit.

This allows me to use CMAKE_BUILD_WITH_INSTALL_RPATH=False and the Ninja
generator on macOS, whereas before it would always give me the following
error message (despite cross-compiling for an ELF-based platform):

```
The install of the <name> target requires changing an RPATH from
  the build tree, but this is not supported with the Ninja generator unless
  on an ELF-based platform.  The CMAKE_BUILD_WITH_INSTALL_RPATH variable may
  be set to avoid this relinking step.
```