Previously setting LIBC_COPT_STRING_UNSAFE_WIDE_READ would cause a build
error because there is a path in the ifdef that doesn't return anything.

The string_utils.h file previously included both memcpy and bzero. There
were no uses of bzero, and only one use of memcpy which was replaced
with __builtin_memcpy.

Also fix strsep which was broken by this change, fix a useless assert of
"sizeof(char) == sizeof(cpp::byte)", and update the bazel.

Relates to
https://github.com/llvm/llvm-project/issues/119281#issuecomment-2699470459

This reverts commit 1e6e845d49a336e9da7ca6c576ec45c0b419b5f6 because it
changed the 1st parameter of adjust() to be unsigned, but libc itself
calls adjust() with a negative argument in align_backward() in
op_generic.h.

Relates to: #119281

Update string_utils' string_length to work with char* or wchar_t*, so that it
may be reusable when implementing wmemchr, wcspbrk, wcsrchr, wcsstr.

Link: #121183
Link: #124027

Co-authored-by: Nick Desaulniers <ndesaulniers@google.com>

---------

Co-authored-by: Tristan Ross <tristan.ross@midstall.com>

`strrchr("foo", '\0')` is defined to point to the end of `foo`, rather
than returning NULL. This wasn't caught by tests, since llvm-libc's
`ASSERT_STREQ(nullptr, "");` is not an assertion error.

While I'm here, refactor the test slightly to check for NULL more
specifically. I considered adding fancier `ASSERT`s (and changing the
semantics of `ASSERT_STREQ`), but opted for a more local fix by fair
dice roll.

When running Bionic's testsuite over llvm-libc, tests broke because
e.g.,

```
const char *str = "abc";
char buf[7]{"111111"};
strlcpy(buf, str, 7);
ASSERT_EQ(buf, {'1', '1', '1', '\0', '\0', '\0', '\0'});
```

On my machine (Debian w/ glibc and clang-16), a `printf` loop over `buf`
gets unrolled into a series of const `printf` at compile-time:
```
printf("%d\n", '1');
printf("%d\n", '1');
printf("%d\n", '1');
printf("%d\n", 0);
printf("%d\n", '1');
printf("%d\n", '1');
printf("%d\n", 0);
```

Seems best to match existing precedent here.

This is a part of #97655.