malloc: Add a fastpath

This diff adds a fastpath that assumes size <= SC_LOOKUP_MAXCLASS, and
that we hit tcache.  If either of these is false, we fall back to
the previous codepath (renamed 'malloc_default').

Crucially, we only tail call malloc_default, and with the same kind
and number of arguments, so that both clang and gcc tail-calling
will kick in - therefore malloc() gets treated as a leaf function,
and there are *no* caller-saved registers.   Previously malloc() contained
5 caller saved registers on x64, resulting in at least 10 extra
memory-movement instructions.

In microbenchmarks this results in up to ~10% improvement in malloc()
fastpath.  In real programs, this is a ~1% CPU and latency improvement
overall.