ICU-20575 fix broken default locale mapping for C.UTF-8

- Partial revert of 1afef30
- PR#418 incorrectly dropped the mapping from C to en-us-POSIX
- For cases where a codepage was set (such as C.UTF-8) the
  hard coded fallback logic in putil.cpp would not activate.
- The “strcmp("C", …)” in putil.cpp is intended to detect specific
  platform behavior, it is not a general mapping.

icu4c/source/common/uloc.cpp

@@ -464,6 +464,8 @@ typedef struct CanonicalizationMap {
  * different semantic kinds of transformations.
  */
 static const CanonicalizationMap CANONICALIZE_MAP[] = {
+    { "c",              "en_US_POSIX" }, /* POSIX name */
+    { "posix",          "en_US_POSIX" }, /* POSIX name (alias of C) */
     { "art_LOJBAN",     "jbo" }, /* registered name */
     { "hy__AREVELA",    "hy" }, /* Registered IANA variant */
     { "hy__AREVMDA",    "hyw" }, /* Registered IANA variant */