cmd/go: enable link syso test on linux/riscv64
cmd/go: enable link syso test on linux/riscv64

Now that external linking is supported on linux/riscv64 (CL 243517),
re-enable the test previously disabled by CL 216259.

Updates #36739

Change-Id: I611548c587ca50f8bfab72c903ab0432e2b54198
Reviewed-on: https://go-review.googlesource.com/c/go/+/263797
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Joel Sing <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>

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