libc/mingw: move content of do_libc_headers into do_libc_start_files
It is unnecessary to split C library preparation into two steps, as only
one really makes sense. So, do_libc_headers is bound to be withdrawn
short-term, in favor of do_libc_start_files.
mingw already had all its start files installation in do_libc_headers, and
do_libc_start_files was empty, just migrate the content of the former into
the latter.
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>