-fno-builtin suppresses optimizations such as turning calls to `sqrt`
or `fabs` into `f64.sqrt` or `f64.abs` instructions inline. Libc code
itself benefits from these optimizations.
I originally added this flag because historically it was needed when
building libc to avoid the compiler pattern-matching the body of memcpy
into a memcpy call, however clang no longer requires this.
* Expand the comment about why we use USE_DL_PREFIX in dlmalloc.