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

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

tests/cpp/texportc.nim

@@ -0,0 +1,22 @@
+discard """
+  targets: "c cpp"
+"""
+
+var fun0 {.importc.}: int
+proc fun1() {.importc.}
+proc fun2() {.importc: "$1".}
+proc fun3() {.importc: "fun3Bis".}
+
+when defined cpp:
+  # proc funx1() {.importcpp.} # this does not work yet
+  proc funx1() {.importc: "_Z5funx1v".}
+
+doAssert fun0 == 10
+fun1()
+fun2()
+fun3()
+
+when defined cpp:
+  funx1()
+
+import ./mexportc