math: add scalar erf

Only tested in round-to-nearest mode. The expected worst case error
is 1.01 ULP near x=1.25.  Benchmarked over random x in [-6,6] and
can increase performance by > 2x (> 3.5x for throughput) on big ooo
cores compared to the implementation in glibc 2.28.

Includes data for erfc too, but this patch only adds erf.