os: add WASI (WebAssembly System Interface).

Signed-off-by: Piotr Sikora <piotrsikora@google.com>

os/BUILD

diff --git a/os/BUILD b/os/BUILD
index 16525d3..53fec81 100644
--- a/os/BUILD
+++ b/os/BUILD
@@ -84,3 +84,10 @@
     name = "nixos",
     constraint_setting = ":os",
 )
+
+# WASI (WebAssembly System Interface)
+# https://github.com/bytecodealliance/wasmtime/blob/main/docs/WASI-overview.md
+constraint_value(
+    name = "wasi",
+    constraint_setting = ":os",
+)