This CL affects the low-level -trimpath flag provided
by both cmd/asm and cmd/compile. Previously, the flag
took the name of a single directory that would be trimmed
from recorded paths in the resulting object file.
This CL makes the flag take a semicolon-separated list of paths.
Further, each path can now end in an optional "=>replacement"
to specify what to replace that leading path prefix with,
instead of only dropping it.
A followup CL will add a mode to cmd/go that uses this
richer -trimpath to build binaries that do not contain any
local path names.