Various fixes in GoCompilePkg (#2045)

* Fix C++ header detection: add hh, hpp and hxx file types.
* Ensure C/C++ flags are set before the compiler inputs and outputs.
  If the language is forced in copts, for instance -x objective-c,
  this ensures that the compiler properly sees it before compiling the source.

Signed-off-by: Steeve Morin <steeve@zen.ly>