ICU-23031 Reinstate special case for "-u-va-posix" lost by ICU-22520 #3500

Job	Run time
clang-valgrind-test	10m 41s
clang-valgrind-intltest (utility)	7m 11s
clang-valgrind-intltest (normalize)	6m 38s
clang-valgrind-intltest (collator)	5m 43s
clang-valgrind-intltest (regex)	6m 4s
clang-valgrind-intltest (format)	17m 7s
clang-valgrind-intltest (translit)	6m 29s
clang-valgrind-intltest (rbbi)	10m 14s
clang-valgrind-intltest (rbnf)	6m 53s
clang-valgrind-intltest (rbnfrt)	6m 44s
clang-valgrind-intltest (icuserv)	5m 45s
clang-valgrind-intltest (idna)	5m 30s
clang-valgrind-intltest (convert)	7m 53s
clang-valgrind-intltest (rbnfp)	5m 59s
clang-valgrind-intltest (csdet)	5m 41s
clang-valgrind-intltest (spoof)	5m 28s
clang-valgrind-intltest (bidi)	5m 46s
	2h 5m 46s

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