Switch to FreeBSD's actual sincos.

Before:
  BM_math_sin_fast                48 ns         48 ns   14693053
  BM_math_sincos                  61 ns         61 ns   11470219

After:
  BM_math_sin_fast                48 ns         48 ns   14725120
  BM_math_sincos                  43 ns         43 ns   16329843

Bug: N/A
Test: ran tests, benchmarks
Change-Id: I8693c64135233c0641af5302c38748f47ac76737