[dev.regabi] all: merge master (1d781391) into dev.regabi

Merge List:

+ 2020-12-26 1d781391 runtime/cgo: fix Android build with NDK 22
+ 2020-12-25 2018b68a net/mail: don't use MDT in test
+ 2020-12-23 b1164044 runtime: shift timeHistogram buckets and allow negative durations
+ 2020-12-23 8db7e2fe runtime: fix allocs-by-size and frees-by-size buckets
+ 2020-12-23 fb96f07e runtime: fix nStackRoots comment about stack roots
+ 2020-12-23 d1502b3c lib/time, time/tzdata: update tzdata to 2020e
+ 2020-12-23 30c99cbb cmd/go: add the Retract field to 'go help mod edit' definition of the GoMod struct
+ 2020-12-23 49d0b239 doc: fix a typo in contribute.html
+ 2020-12-23 98a73030 cmd/go: in 'go get', promote named implicit dependencies to explicit
+ 2020-12-23 fd6ba1c8 os/signal: fix a deadlock with syscall.AllThreadsSyscall() use
+ 2020-12-23 b0b0d982 runtime: linux iscgo support for not blocking nptl signals
+ 2020-12-22 223331fc cmd/go/internal/modload: add hint for missing implicit dependency

Change-Id: I76d79f17c546cab03fab1facc36cc3f834d9d126