cmd/compile,cmd/internal/obj/riscv: load >32-bit constants from memory for riscv64

Follow what MIPS does and load >32-bit constants from memory using two instructions,
rather than generating a four to six instruction sequence. This removes more than 2,500
instructions from the Go binary. This also makes it possible to load >32-bit constants
via a single assembly instruction, if required.

Change-Id: Ie679a0754071e6d8c52fe0d027f00eb241b3a758
Reviewed-on: https://go-review.googlesource.com/c/go/+/302609


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Zhang <cherryyz@google.com>