Bug 1673769 - Use the right data file for ICU on big endians. r=firefox-build-system-reviewers,dmajor

It turns out that CONFIG.get(..., ...) doesn't actually work properly in
moz.build, so use `or` instead.

Differential Revision: https://phabricator.services.mozilla.com/D94918

config/external/icu/data/moz.build

@@ -24,7 +24,7 @@ data_dir = {
     "little": SRCDIR,
     "big": OBJDIR,
 }
-endianness = CONFIG.get("TARGET_ENDIANNESS", "little")
+endianness = CONFIG["TARGET_ENDIANNESS"] or "little"
 DEFINES["ICU_DATA_FILE"] = '"%s/%s"' % (data_dir[endianness], data_file[endianness])
 DEFINES["ICU_DATA_SYMBOL"] = "%sicudt%s_dat" % (prefix, CONFIG["MOZ_ICU_VERSION"])
 SOURCES += [