[libc] move bcmp, bzero, bcopy, index, rindex, strcasecmp, strncasecmp to strings.h (#118899)

docgen relies on the convention that we have a file foo.cpp in
libc/src/\<header\>/. Because the above functions weren't in libc/src/strings/
but rather libc/src/string/, docgen could not find that we had implemented
these.

Rather than add special carve outs to docgen, let's fix up our sources for
these 7 functions to stick with the existing conventions the rest of the
codebase follows.

Link: #118860
Fixes: #118875

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