port newlib's asinf

[japaric]

Source: https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=newlib/libm/mathfp/sf_asine.c;hb=HEAD

Rationale: the current implementation makes use of f64 and it has bad performance on architectures that have no hardware support for double precision floats.

What needs to be done:

• Move the existing MUSL port into a private math::musl module and make sure we continue compiling it.
• Port the newlib version into src/math/$fun.rs
• Update the test suite by uncommenting the function name in tests/newlib.rs and removing the function name from tests/musl.rs.

[burrbull]

#141

[m1el]

newlib's asinf is using f64 on line 66

       y = -2 * sqrt (g); <-- sqrt is f64->f64

[burrbull]

You are right.

[vks]

I wonder whether it is intentional.