compilebench: add a linker benchmark
compilebench: add a linker benchmark

This links cmd/compile, which is the largest binary in GOROOT.

Unfortunately, this can't measure the linker's allocation metrics
prior to CL 176057 because the fix for golang/go#18641 was applied to
the compiler but not the linker.

Change-Id: Ia44d80f1d727c1608cedaa4b8ad2a05903774875
Reviewed-on: https://go-review.googlesource.com/c/tools/+/175801
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools