Make x86 use the C versions of bcmp() and memcmp().

No need for bzero, memset provides it.

Remove now-non-existant bzero.S from sources list.

branches: 1.4.6; 1.4.12;
If SRCS contains any .S files then remove the corresponding .c file from
SRCS and add to LSRCS (for lint).
Change the 'string' Makefiles so that the arch/*/string/Makefile.inc
need only specify the .S files for that architecture and not the .c
files for all the files they don't override.

Change all archs so that strchr.[cS] and strrchr.[cS] exist and generate
duplicate symbols for index() and rindex().
libc and kernel build ok for affected archs (i386, amd64, vax, sun3,
evbmips-eb) all seem ok.
(vax might lose an asm index() - but comments imply it is worse than the C)

Rework how dependency generation is performed:

* DPSRCS contains extra dependencies, but is _NOT_ added to CLEANFILES.
This is a change of behaviour. If a Makefile wants the clean semantics
it must specifically append to CLEANFILES.
Resolves PR toolchain/5204.

* To recap: .d (depend) files are generated for all files in SRCS and DPSRCS
that have a suffix of: .c .m .s .S .C .cc .cpp .cxx

* If YHEADER is set, automatically add the .y->.h to DPSRCS & CLEANFILES

* Ensure that ${OBJS} ${POBJS} ${LOBJS} ${SOBJS} *.d depend upon ${DPSRCS}

* Deprecate the (short lived) DEPENDSRCS


Update the various Makefiles to these new semantics; generally either
adding to CLEANFILES (because DPSRCS doesn't do that anymore), or replacing
specific .o dependencies with DPSRCS entries.

Tested with "make -j 8 distribution" and "make distribution".

Initial commit of x86-64 specific parts of libc.