[dev.typeparams] all: merge master (eb982727) into dev.typeparams

Merge List:

+ 2021-02-18 eb982727 cmd/go/internal/mvs: fix Downgrade to match Algorithm 4
+ 2021-02-18 3b7277d3 cmd/go: add a script test for artifacts resulting from 'go get -u'
+ 2021-02-18 f3c2208e cmd/go: add script tests for potential upgrades due to downgrades
+ 2021-02-18 a5c8a15f cmd/go/internal/mvs: clarify and annotate test cases
+ 2021-02-18 a76efea1 cmd/go/internal/mvs: don't emit duplicates from Req
+ 2021-02-18 609d82b2 cmd/dist: set GOARM=7 for windows/arm
+ 2021-02-18 f0be3cc5 runtime: unbreak linux/riscv64 following regabi merge
+ 2021-02-18 07ef3135 runtime/cgo: add cast in C code to avoid C compiler warning

Change-Id: I8e58ad1e82a9ea313a99c1b11df5b341f80680d4