[dev.link] cmd/internal/goobj2, cmd/link: experiment another way of accessing relocations

[dev.link] cmd/internal/goobj2, cmd/link: experiment another way of accessing relocations

Use a different mechanism to access relocations from the object
files, and use it in the stack bounds check pass. This shows some
speedup.

(linking cmd/compile)
Dostkcheck     76.9ms ± 1%    55.1ms ± 1%  -28.36%  (p=0.008 n=5+5)

Change-Id: I2ac42da515dccd64719fb557ffff6cdc69e4319b
Reviewed-on: https://go-review.googlesource.com/c/go/+/222240
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>