Fix exception specifiers

Per
https://status-im.github.io/nim-style-guide/interop.c.html#functions-and-types,
imported functions must be annotated with exception information.

If this is not done, Nim assumes the worst case that the function
potentially could raise an exception which breaks compile-time exception
safety analysis.

status_go/impl.nim

@@ -1,3 +1,6 @@
+# go functions do not raise nim exceptions and do not interact with the Nim gc
+{.push raises: [], gcsafe.}
+
 type SignalCallback* = proc(signal: cstring): void {.cdecl.}
 
 # All procs start with lowercase because the compiler will also need to import