10x printf speedup.

Android is UTF-8. Don't make everyone pay to convert UTF-8 to ASCII just
so we can recognize '%'. With UTF-8 we can just strchr forwards.

Before:

  ---------------------------------------------------------------
  Benchmark                        Time           CPU Iterations
  ---------------------------------------------------------------
  BM_stdio_printf_literal       1290 ns       1290 ns     442554
  BM_stdio_printf_s             1204 ns       1204 ns     582446
  BM_stdio_printf_d             1206 ns       1206 ns     578311
  BM_stdio_printf_1$s           2263 ns       2263 ns     310002

After:

  ---------------------------------------------------------------
  Benchmark                        Time           CPU Iterations
  ---------------------------------------------------------------
  BM_stdio_printf_literal        178 ns        178 ns    3394001
  BM_stdio_printf_s              246 ns        246 ns    2850284
  BM_stdio_printf_d              252 ns        252 ns    2778610
  BM_stdio_printf_1$s            363 ns        363 ns    1929011

Add missing __find_arguments error checking to the wide variant to match
the regular one.

Also replace various char/wchar_t differences with the macro.

Bug: http://b/67371539
Test: ran tests
Change-Id: I18f122009c22699943ab5d666a98ea594a972c40