Add calling convention macros to asm_defines to be used in asm functions

Instead of push/pop #if/#else by architecture and system in asm code, provide proper PUSH_CALLEE_SAVED_REGS/POP_CALLEE_SAVED_REGS macros that can be used instead.

Start by locating asm_defines and the assembly functions that currently duplicate architecture- and system-specific push/pop conditionals. Define the requested callee-saved-register macros there, replace the duplicated conditionals in the affected assembly functions, and verify that the relevant assembly builds still work across the supported architectures and systems.

Written by the indexing model from the issue text.