C/C++: Use single-precision variants of math functions when appropriate

[KGHustad]

This PR fixes a number of issues with generating single-precision code. The issues fall into two main categories:

• Float literals lacking an f suffix.
• Math library function calls returning a double value. These have now been replaced with the equivalent single-precision variant.

I used https://en.wikipedia.org/wiki/C_mathematical_functions for an exhaustive list of math functions in C and filtered out those without single-precision variants.

Additionally, I have added a special case to rewrite (exp(a) - 1) as (expm1(a)) to increase the precision when this expression is near zero.