Fix loading of libgccjit.dll while dumping in Windows.

Fix loading of libgccjit.dll while dumping in Windows.

loadup.el calls `native-comp-available-p', that calls
load_gccjit_if_necessary() in Windows. That function tries to load
libgccjit using the mappings defined in `dynamic-library-alist'. That
mapping is filled by term/w32-win.el, but that file may be loaded too
late.

* src/emacs.c (syms_of_emacs): Add libgccjit to the
`dynamic-library-alist' used when starting to dump so
`native-comp-available-p' always works in Windows.