exportc now mangles same as importc, fixes #10578 (#12144)

* fixes #10578
* add tests
* add changelog
* add {.exportcpp.}

tests/cpp/mexportc.nim

@@ -0,0 +1,9 @@
+{.used.} # ideally, would not be needed
+
+var fun0 {.exportc.} = 10
+proc fun1() {.exportc.} = discard
+proc fun2() {.exportc: "$1".} = discard
+proc fun3() {.exportc: "fun3Bis".} = discard
+
+when defined cpp:
+  proc funx1() {.exportcpp.} = discard