port newlib's exp2f

[japaric]

Source: https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=newlib/libm/math/wf_exp2.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.

[tesuji]

Look like someone already do it.

[tgross35]

I am closing all newlib issues; please see #133 (comment) for context.