Bits and pieces on the bottom of the module dependency tree. Also import the required constants, so we know what we're using.
In the interests of cross-compilation, we want to free ourselves from the autoconf generated modules like main/Constants
We (allegedly) put the first six C-call arguments in registers; where do we start putting the rest of them?
Check whether an integer will fit in 32 bits. A CmmInt is intended to be truncated to the appropriate number of bits, so here we truncate it to Int64. This is important because e.g. -1 as a CmmInt might be either -1 or 18446744073709551615.