all: add support for c-archive and c-shared on linux/riscv64

This provides the runtime glue (_rt0_riscv64_linux_lib) for c-archive and c-shared
support, along with enabling both of these buildmodes on linux/riscv64.

Both misc/cgo/testcarchive and misc/cgo/testcshared now pass on this platform.

Fixes #47100

Change-Id: I7ad75b23ae1d592dbac60d15bba557668287711f
Reviewed-on: https://go-review.googlesource.com/c/go/+/334872


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