[dev.regabi] all: merge master (c9fb4eb0) into dev.regabi

Merge List:

* 2020-12-22 c9fb4eb0 cmd/link: handle grouped resource sections
* 2020-12-22 c06a354b test: trigger SIGSEGV instead of SIGTRAP in issue11656.go
* 2020-12-22 0aa9b470 cmd/pack: r command create output file if not exist
* 2020-12-22 4d27c4c2 runtime: correct error handling in several FreeBSD syscall wrappers
* 2020-12-22 9b614712 cmd/pack: treat compiler's -linkobj output as "compiler object"
* 2020-12-21 bc7e4d92 syscall: don't generate ptrace on iOS
* 2020-12-21 6cff874c runtime/metrics: add Read examples
* 2020-12-21 8438a577 runtime: use _exit on darwin
* 2020-12-21 cb95819c runtime: detect netbsd netpoll overrun in sysmon
* 2020-12-21 53c984d9 runtime: skip wakep call in wakeNetPoller on Plan 9
* 2020-12-21 9abbe277 test: skip issue11656.go on mips/mips64/ppc64

Change-Id: Ibb235fbf6a86ebcf50c686dc11f7c02d1865f845