libc: strlcpy/strlcat shouldn't bzero the rest of `buf` (#114259)

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.

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