cmd/internal/obj: add prologue_end DWARF stmt for riscv64

cmd/internal/obj: add prologue_end DWARF stmt for riscv64

This patch adds prologue_end statement to the DWARF info for riscv64,
which delve debugger uses for skip stacksplit prologue.

Change-Id: I4e5d9c26202385f65b3118b16f53f66de9d327f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/620295
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>