ICU-20570 Fix data race in initializeFCD (usearch.cpp) via UInitOnce

initializeFCD may be invoked concurrently from multiple threads, so the
simple null-check on g_nfcImpl is a data race. Replace it with
umtx_initOnce, which is designed exactly for this use case and is the
pattern used throughout ICU.

Ported from snowflake-eng/snowflake#461615.

Author: sfc-gh-bhannel

icu4c/source/i18n/usearch.cpp

@@ -22,6 +22,7 @@
 #include "cmemory.h"
 #include "ucln_in.h"
 #include "uassert.h"
+#include "umutex.h"
 #include "ustr_imp.h"
 
 U_NAMESPACE_USE
@@ -33,6 +34,7 @@ U_NAMESPACE_USE
 #define SUPPLEMENTARY_MIN_VALUE_ 0x10000
 
 static const Normalizer2Impl *g_nfcImpl = nullptr;
+static UInitOnce gSearchNFCInitOnce = U_INITONCE_INITIALIZER;
 
 // internal methods -------------------------------------------------
 
@@ -76,10 +78,16 @@ U_CDECL_BEGIN
 static UBool U_CALLCONV
 usearch_cleanup() {
     g_nfcImpl = nullptr;
+    gSearchNFCInitOnce.reset();
     return true;
 }
 U_CDECL_END
 
+static void U_CALLCONV initNFCImpl(UErrorCode *status) {
+    g_nfcImpl = Normalizer2Factory::getNFCImpl(*status);
+    ucln_i18n_registerCleanup(UCLN_I18N_USEARCH, usearch_cleanup);
+}
+
 /**
 * Initializing the fcd tables.
 * Internal method, status assumed to be a success.
@@ -89,10 +97,7 @@ U_CDECL_END
 static
 inline void initializeFCD(UErrorCode *status)
 {
-    if (g_nfcImpl == nullptr) {
-        g_nfcImpl = Normalizer2Factory::getNFCImpl(*status);
-        ucln_i18n_registerCleanup(UCLN_I18N_USEARCH, usearch_cleanup);
-    }
+    umtx_initOnce(gSearchNFCInitOnce, &initNFCImpl, status);
 }
 
 /**