compiler/ccgtypes: hide exportc proc unless it has dynlib (#13199)

This hides most of stdlib's internal functions from resulting
binaries/libraries, where they aren't needed on *nix. Static libraries
are not affected by this change (visibility doesn't apply to them).

tests/dll/visibility.nim

@@ -0,0 +1,19 @@
+discard """
+  output: "could not import: foo"
+  exitcode: 1
+"""
+
+const LibName {.used.} =
+  when defined(windows):
+    "visibility.dll"
+  elif defined(macosx):
+    "libvisibility.dylib"
+  else:
+    "libvisibility.so"
+
+when compileOption("app", "lib"):
+  proc foo() {.exportc.} =
+    echo "failed"
+elif isMainModule:
+  proc foo() {.importc, dynlib: LibName.}
+  foo()