Fix reporting of callee saved registers with nongnu libuwind

Fix reporting of callee saved registers with nongnu libuwind

The callee saved registers were not preserved for GC stackwalk with nongnu
libunwind that does not provide context pointers. Fixed by adding an extra
space to MachState to preserve them.

To avoid large amounts of copy&pasted code, introduced macro to enumerate
all callee saved registers, similar to existing macro to enumerate all
argument registers. This macro makes the code to copy the register values
between different structures more compact, and less prone to typos.