cmd/link: fix deferreturn detector

[randall77]

The logic for detecting deferreturn calls is wrong.
We look for a relocation whose symbol is runtime.deferreturn and has an offset of 0. But on some architectures, the relocation offset is not zero. These include arm (the offset is 0xebfffffe) and s390x (the offset is 6).
This ends up having the deferreturn offset at 0, so we end up using the entry point live map instead of the deferreturn live map in a frame which defers and then segfaults.

Update #6980

[gopherbot]

Change https://golang.org/cl/181258 mentions this issue: cmd/link: fix deferreturn detector

[randall77]

@gopherbot, please open a backport to 1.12.

[gopherbot]

Backport issue(s) opened: #32484 (for 1.12).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.

[gopherbot]

Change https://golang.org/cl/181378 mentions this issue: Revert "Revert "cmd/compile,runtime: allocate defer records on the stack""