Improve performance of __libc_malloc by splitting it into 2 parts: first handle the tcache fastpath, then do the rest in a separate tailcalled function. This results in significant performance gains since __libc_malloc doesn't need to setup a frame and we delay tcache initialization and setting of errno until later. On Neoverse V2, bench-malloc-simple improves by 6.7% overall (up to 8.5% for ST case) and bench-malloc-thread improves by 20.3% for 1 thread and 14.4% for 32 threads. Reviewed-by: DJ Delorie <dj@redhat.com>