[dev.typeparams] all: merge master (ecaa6816) into dev.typeparams

Conflicts:

- src/cmd/compile/internal/ssagen/ssa.go

  CL 336629 touched code that had already been removed on dev.typeparams.

Merge List:

+ 2021-07-26 ecaa6816 doc: clarify non-nil zero length slice to array pointer conversion
+ 2021-07-26 1868f829 crypto/x509: update iOS bundled roots to version 55188.120.1.0.1
+ 2021-07-25 849b7911 spec: use consistent capitalization for rune literal hex constants
+ 2021-07-23 0914646a doc/1.17: fix two dead rfc links
+ 2021-07-22 052da571 cmd/compile: do not change field offset in ABI analysis

Change-Id: Ie570ec3f6a3241e0495e39e8a73b3a09a9368605