defface in subr.el breaks bootstrap/build

[Tim Van Holder]

Revision 1.541 of subr.el introduced a new face.
However, subr.el is loaded so early during emacs' dump that neither
defface nor make-face are available - as a result building emacs
is no longer possible (regular builds error out with
'custom-declare-face' not defined; bootstrap builds complain about
'defface' instead).  Moving the defface for 'momentary' to faces.el
allows the build to work again.