MinGW: Implement ldexp in asm for true 80 bit precision.

An implementation that uses an (inlinable, ...) inline asm
expression would be much preferred.
MinGW: core.stdc.math macro implementations.
MinGW: Use __mingw_* versions of C formatting functions.

The unprefixed versions are pulled in from the Microsoft C
runtime by default, and thus do not support long doubles
and, in the case of ldexp, the hexadecimal floating point
literal syntax.
MinGW: Fix TLS range detection.

This piece of code will need adjustment for Win64/MSVC as well!