build: all.bash takes far too long

[rsc]

all.bash is taking too long. There are many reasons. This is a general tracking bug for work to fix this.

[josharian]

One regression: The os/exec race detector test in Go 1.4.2 went from ~0.6s on my machine to ~17.5s with tip. all.bash went from ~2.5m to ~6m, so this accounts for about 15% of the regression.

/cc @dvyukov

[gopherbot]

CL https://golang.org/cl/9350 mentions this issue.

[gopherbot]

CL https://golang.org/cl/9913 mentions this issue.

[gopherbot]

CL https://golang.org/cl/10211 mentions this issue.