Fix impl of ReadOnlySpan ToLower/ToUpper for Unix.

[ahsonkhan]

Update to #16379

Ran the new tests locally on OSX and they all pass now.

Fixes the test failures - https://mc.dot.net/#/user/ahsonkhan/pr~2Fjenkins~2Fdotnet~2Fcorefx~2Fmaster~2F/test~2Ffunctional~2Fcli~2F/e0a26daab80419c10a37646444d5155d5495d241/workItem/System.Memory.Tests/wilogs

System.SpanTests.ReadOnlySpanTests.LengthMismatchToLower [FAIL]
2018-02-22 03:47:55,702: INFO: proc(54): run_and_log_output: Output:       Assert.Equal() Failure
2018-02-22 03:47:55,703: INFO: proc(54): run_and_log_output: Output:       Expected: Char[] ['a', 'b', 'c', 'D']
2018-02-22 03:47:55,703: INFO: proc(54): run_and_log_output: Output:       Actual:   Char[] ['a', 'b', 'c', '\0']

Related PR: dotnet/corefx#27193

cc @jkotas, @stephentoub, @KrzysztofCwalina, @tarekgh, @JeremyKuhne

[ahsonkhan]

@dotnet-bot test Ubuntu arm64 Cross Debug Innerloop Build

[ahsonkhan]

@dotnet-bot test Ubuntu x64 Checked Innerloop Build and Test