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

This test requires support for external linking, which does not currently
exist on linux/riscv64.

Updates #27532 and #36739

Change-Id: Ia0cdf69f1830b995f2882b47d1bc0be82c4b3039
Reviewed-on: https://go-review.googlesource.com/c/go/+/216259
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: 4a6f656c

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

@@ -8,6 +8,10 @@
 # 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
+
 # External linking is not supported on darwin/386 (10.14+).
 # See: https://github.com/golang/go/issues/31751
 [darwin] [386] skip