cmd/go: `TestScript/gccgo_link_c` failures on illumos/amd64 and aix/ppc64

[mknyszek]

The immediate culprit appears to be https://go.dev/cl/416134.

CC @bcmills @rsc

[mknyszek]

Example failures:

https://build.golang.org/log/a368c7af5e59ff8dbf7ad719542fa970acac7e2a
https://build.golang.org/log/17e8cff16241ef66df2595817b861b7fadba309a

[gopherbot]

Change https://go.dev/cl/417094 mentions this issue: cmd/go: avoid indexing GOROOT packages when the compiler is 'gccgo'

[bcmills]

Already had a fix pending; added the issue# to it.