runtime: build fastlog

Take out the build tags which were preventing fastlog2 from being
built.  It's used by the upcoming Go 1.8 GC.

Change-Id: Ieb2e1e3b4d907967142a3b2a235700c35e795bca
Reviewed-on: https://go-review.googlesource.com/40773
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>