cmd/compile: implement atomic intrinsics for riscv64

[4a6f656c]

Go on riscv64 does not currently support atomic intrinsics, rather all atomics are functions. This should be implemented.

[gopherbot]

Change https://golang.org/cl/216263 mentions this issue: test: adjust tests for riscv64

[cagedmantis]

/cc @randall77 @griesemer

[gopherbot]

Change https://golang.org/cl/220540 mentions this issue: cmd/internal/obj/riscv: add LR/SC instructions

[gopherbot]

Change https://golang.org/cl/220541 mentions this issue: cmd/internal/obj/riscv: add atomic memory operation instructions

[gopherbot]

Change https://golang.org/cl/220539 mentions this issue: cmd/internal/obj/riscv: add FENCE instruction

[gopherbot]

Change https://golang.org/cl/223557 mentions this issue: cmd/compile: provide Load8/Store8 atomic intrinsics on riscv64

[gopherbot]

Change https://golang.org/cl/223560 mentions this issue: test: re-enable atomic intrinsic related tests on riscv64

[gopherbot]

Change https://golang.org/cl/223558 mentions this issue: cmd/compile: provide Load32/Load64/Store32/Store64 atomic intrinsics on riscv64