Make explicit {.nimcall.} a seperate calling convention

tests/closure/tinvalidclosure.nim

@@ -1,5 +1,5 @@
 discard """
-  errormsg: "type mismatch: got <proc (x: int){.gcsafe, locks: 0.}>"
+  errormsg: "type mismatch: got <proc (x: int){.nimcall, gcsafe, locks: 0.}>"
   line: 12
 """
 

tests/closure/tinvalidclosure4.nim

@@ -0,0 +1,9 @@
+discard """
+  errormsg: "illegal capture 'v'"
+  line: 7
+"""
+
+proc outer(v: int) =
+  proc b {.nimcall.} = echo v
+  b()
+outer(5)