Speed up __sfileext initialization.

The internal uses don't need to actually initialize a mutex since they'll
never escape and don't need locking. It's a small saving, but easy.

Before:

  BM_stdio_scanf_d                    465 ns        465 ns    1507891
  BM_stdio_scanf_maps                1836 ns       1836 ns     381082
  BM_stdio_scanf_maps_baseline        846 ns        845 ns     830881
  BM_stdio_scanf_s                    419 ns        419 ns    1671979

After:

  BM_stdio_scanf_d                    434 ns        434 ns    1612930
  BM_stdio_scanf_maps                1815 ns       1815 ns     386470
  BM_stdio_scanf_maps_baseline        875 ns        873 ns     803304
  BM_stdio_scanf_s                    382 ns        382 ns    1833198

Bug: http://b/70862888
Test: ran tests, benchmarks
Change-Id: Ic8822aaca5d8ca1a73390089153d0fe35d91673e