[libc] clean up string_utils memory functions (#143031)
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.
