Call and link executables directly in INSTDIR.

- gb_Library_get_target is now same as the gb_LinkTarget_get_target
- disable gb_Library_add_auxtarget, no auxtargets need to be copied
- adjust paths of all external executables to OUTDIR_FOR_BUILD for now
- use lazy assignment instead of := in AllLangResTarget because it's
  read before Executable
- link.exe generates an import library for lots of executables
  because they export symbols, especially since commit
  0ffab936
  "force 'main' to always be DLLPUBLIC."

Change-Id: I3e1ee7425dd430bb83c7cd59e265869a0541b38d