cmd/go: TestScript/build_trimpath broken with gccgo 10.2.1 as of CL 266365

[bcmills]

+ /usr/local/google/home/bcmills/go-review/bin/go test cmd/go -run=TestScript/build_trimpath
go test proxy running at GOPROXY=http://127.0.0.1:39195/mod
--- FAIL: TestScript (0.01s)
    --- FAIL: TestScript/build_trimpath (4.42s)
        script_test.go:213:
            # If GOROOT_FINAL is set, 'go build -trimpath' bakes that into the resulting
            # binary instead of GOROOT. Explicitly unset it here. (0.000s)
            # Set up two identical directories that can be used as GOPATH. (0.001s)
            # A binary built without -trimpath should contain the module root dir
            # and GOROOT for debugging and stack traces. (0.217s)
            # A binary built with -trimpath should not contain the current workspace
            # or GOROOT. (2.437s)
            # A binary from an external module built with -trimpath should not contain
            # the current workspace or GOROOT. (0.356s)
            # Two binaries built from identical packages in different directories
            # should be identical. (0.182s)
            # Same sequence of tests but with overlays.
            # A binary built without -trimpath should contain the module root dir
            # and GOROOT for debugging and stack traces. (0.212s)
            # A binary built with -trimpath should not contain the current workspace
            # or GOROOT. (0.202s)
            # Two binaries built from identical packages in different directories
            # should be identical. (0.158s)
            # Same sequence of tests but in GOPATH mode.
            # A binary built without -trimpath should contain GOPATH and GOROOT. (0.195s)
            # A binary built with -trimpath should not contain GOPATH or GOROOT. (0.202s)
            # Two binaries built from identical packages in different GOPATH roots
            # should be identical. (0.173s)
            # Same sequence of tests but with gccgo.
            # gccgo does not support builds in module mode. (0.000s)
            # A binary built with gccgo without -trimpath should contain the current
            # GOPATH and GOROOT. (0.065s)
            > go build -compiler=gccgo -o paths-dbg.exe paths
            [stderr]
            # paths
            a/src/paths/paths.go:15:18: error: reference to undefined identifier 'os.ReadFile'
               15 |  data, err := os.ReadFile(exe)
                  |                  ^
            [exit status 2]
            FAIL: testdata/script/build_trimpath.txt:101: unexpected command failure

FAIL
FAIL    cmd/go  6.606s

~$ gccgo --version
gccgo (Debian 10.2.1-3) 10.2.1 20201224

I suspect that this was broken by CL 266365 and masked by #35786 (CC @golang/release).

@ianlancetaylor, do you think it's worth backporting a fix to the 1.16 release branch?

CC @jayconrod @matloob

[bcmills]

Confirmed the root cause with git bisect. Fix forthcoming.

[gopherbot]

Change https://golang.org/cl/287613 mentions this issue: cmd/go: revert TestScript/build_trimpath to use ioutil.ReadFile

[ianlancetaylor]

This commit will automatically wind up on the 1.16 branch anyhow.

It's useful for tests to pass with gccgo but I think it's not essential. In general I wouldn't worry about backports. Thanks.