cmd/compile/internal: intrinsify publicationBarrier on riscv64

cmd/compile/internal: intrinsify publicationBarrier on riscv64

This enables publicationBarrier to be used as an intrinsic
on riscv64, optimizing the required function call and return
instructions for invoking the "runtime.publicationBarrier"
function.

This function is called by mallocgc. The benchmark results for malloc tested on Lichee-Pi-4A(TH1520, RISC-V 2.0G C910 x4) are as follows.

goos: linux
goarch: riscv64
pkg: runtime
                    │   old.txt   │              new.txt               │
                    │   sec/op    │   sec/op     vs base               │
Malloc8-4             92.78n ± 1%   90.77n ± 1%  -2.17% (p=0.001 n=10)
Malloc16-4            156.5n ± 1%   151.7n ± 2%  -3.10% (p=0.000 n=10)
MallocTypeInfo8-4     131.7n ± 1%   130.6n ± 2%       ~ (p=0.165 n=10)
MallocTypeInfo16-4    186.5n ± 2%   186.2n ± 1%       ~ (p=0.956 n=10)
MallocLargeStruct-4   1.345µ ± 1%   1.355µ ± 1%       ~ (p=0.093 n=10)
geomean               216.9n        214.5n       -1.10%

Change-Id: Ieab6c02309614bac5c1b12b5ee3311f988ff644d
Reviewed-on: https://go-review.googlesource.com/c/go/+/531719
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: M Zhuo <mzh@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Joel Sing <joel@sing.id.au>