sim: m32r: add __linux__ hack for non-Linux hosts

sim: m32r: add __linux__ hack for non-Linux hosts

The m32r Linux syscall emulation logic assumes the host environment
directly matches -- it's being run on 32-bit little endian Linux.
This breaks building for non-Linux systems, so put all the code in
__linux__ ifdef checks.  This code needs a lot of love to make it
work everywhere, but let's at least unbreak it for non-Linux hosts.