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 <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Joel Sing <[email protected]>

Author: tklauser

src/cmd/go/testdata/script/link_syso_issue33139.txt

@@ -8,10 +8,6 @@
 # See: https://github.com/golang/go/issues/8912
 [linux] [ppc64] skip
 
-# External linking is not supported on linux/riscv64.
-# See: https://github.com/golang/go/issues/36739
-[linux] [riscv64] skip
-
 cc -c -o syso/objTestImpl.syso syso/src/objTestImpl.c
 go build -ldflags='-linkmode=external' ./cmd/main.go