newlib: delete (most) redundant lib_a_CCASFLAGS=$(AM_CCASFLAGS)

newlib: delete (most) redundant lib_a_CCASFLAGS=$(AM_CCASFLAGS)

Since automake already sets per-library CCASFLAGS to $(AM_CCASFLAGS)
by default, there's no need to explicitly set it here.

Many of these dirs don't have .S files in the first place, so the rule
doesn't even do anything.  That can easily be seen when Makefile.in has
no changes as a result.

For the dirs with .S files, the custom rules are the same as the pattern
.S.o rules, so this is a nice cleanup.

The only dir that was adding extra flags (newlib/libc/machine/mn10300/)
to the per-library setting can have it moved to the global AM_CCASFLAGS
since the subdir only has one target.  Although the setting just adds
extra debugging flags, so maybe it should be deleted in general.

There are a few dirs that we leave the redundant setting in place.  This
is to workaround an automake limitation in subdirs that support building
with & w/out libtool:
https://www.gnu.org/software/automake/manual/html_node/Objects-created-both-with-libtool-and-without.html