runtime: add Benchmark[Clear|Copy]Fat[16|24]

runtime: add Benchmark[Clear|Copy]Fat[16|24]

These benchmarks are important for performance. When compiling the stdlib:

* 77.1% of the calls to sgen (copyfat) are for 16 bytes; another 8.7% are for 24 bytes. (The next most common is 32 bytes, at 5.7%.)
* Over half the calls to clearfat are for 16 or 24 bytes.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews
https://golang.org/cl/111350043