ICU-21589 Remove format-overflow warning

1. double size one stack buffer
2. add pragma to ignore some theorical case because the array are in
   same length but some fields are shorter than the other.

ICU-21589 wrap with defined(__GNUC__)

ICU-21589 avoid clang

icu4c/source/tools/pkgdata/pkgdata.cpp

@@ -923,6 +923,10 @@ static int32_t initializePkgDataFlags(UPKGOptions *o) {
 static void createFileNames(UPKGOptions *o, const char mode, const char *version_major, const char *version, const char *libName, UBool reverseExt, UBool noVersion) {
     const char* FILE_EXTENSION_SEP = uprv_strlen(pkgDataFlags[SO_EXT]) == 0 ? "" : ".";
     const char* FILE_SUFFIX = pkgDataFlags[LIB_EXT_ORDER][0] == '.' ? "." : "";
+#if defined(__GNUC__) && !defined(__clang__)
+    _Pragma("GCC diagnostic push")
+    _Pragma("GCC diagnostic ignored \"-Wformat-truncation\"")
+#endif
 
 #if U_PLATFORM == U_PF_MINGW
         /* MinGW does not need the library prefix when building in dll mode. */
@@ -1036,6 +1040,10 @@ static void createFileNames(UPKGOptions *o, const char mode, const char *version
               fprintf(stdout, "# libFileName[LIB_FILE_VERSION] = %s  (static)\n", libFileNames[LIB_FILE_VERSION]);
             }
         }
+#if defined(__GNUC__) && !defined(__clang__)
+    _Pragma("GCC diagnostic pop")
+#endif
+
 }
 
 /* Create the symbolic links for the final library file. */
@@ -1132,7 +1140,7 @@ static int32_t pkg_createSymLinks(const char *targetDir, UBool specialHandling)
 
 static int32_t pkg_installLibrary(const char *installDir, const char *targetDir, UBool noVersion) {
     int32_t result = 0;
-    char cmd[SMALL_BUFFER_MAX_SIZE];
+    char cmd[SMALL_BUFFER_MAX_SIZE * 2];
 
     auto ret = snprintf(cmd,
             sizeof(cmd),
@@ -1309,11 +1317,19 @@ static int32_t pkg_archiveLibrary(const char *targetDir, const char *version, UB
      * archive file suffix is the same, then the final library needs to be archived.
      */
     if (uprv_strcmp(pkgDataFlags[SOBJ_EXT], pkgDataFlags[SO_EXT]) != 0 && uprv_strcmp(pkgDataFlags[A_EXT], pkgDataFlags[SO_EXT]) == 0) {
+#if defined(__GNUC__) && !defined(__clang__)
+    _Pragma("GCC diagnostic push")
+    _Pragma("GCC diagnostic ignored \"-Wformat-truncation\"")
+#endif
+
         snprintf(libFileNames[LIB_FILE_VERSION], sizeof(libFileNames[LIB_FILE_VERSION]), "%s%s%s.%s",
                 libFileNames[LIB_FILE],
                 pkgDataFlags[LIB_EXT_ORDER][0] == '.' ? "." : "",
                 reverseExt ? version : pkgDataFlags[SO_EXT],
                 reverseExt ? pkgDataFlags[SO_EXT] : version);
+#if defined(__GNUC__) && !defined(__clang__)
+    _Pragma("GCC diagnostic pop")
+#endif
 
         snprintf(cmd, sizeof(cmd), "%s %s %s%s %s%s",
                 pkgDataFlags[AR],