cmd/dist: don't copy riscv64 specific files for bootstrap build
cmd/dist: don't copy riscv64 specific files for bootstrap build

For now this will only avoid copying math/big/arith_riscv64.s

Change-Id: Ib236e4bf1a6a758649629268a6f512f307596e74
Reviewed-on: https://go-review.googlesource.com/c/go/+/231298
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cloudflare/go,akutz/go,golang/go,CAFxX/go,nawawi/go,golang/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,golang/go,golang/go,CAFxX/go,nawawi/go,akutz/go,akutz/go,CAFxX/go,nawawi/go,akutz/go,nawawi/go,nawawi/go,cloudflare/go,akutz/go,akutz/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,golang/go,akutz/go,CAFxX/go