xen/arm32: head Split and move MMU-specific head.S to mmu/head.S

The MMU specific code in head.S will not be used on MPU systems.
Instead of introducing more #ifdefs which will bring complexity
to the code, move MMU related code to mmu/head.S and keep common
code in head.S. Few notes while moving:
 - As "fail" in original head.S is very simple and this name is too
   easy to be conflicted, duplicate it in mmu/head.S instead of
   exporting it.
 - Realigned ".macro ret" so that the alignment matches to the other
   macros.
 - Rename puts to asm_puts, putn to asm_putn (this denotes that the
   macros are used within the context of assembly only).
 - Use ENTRY() for enable_secondary_cpu_mm, enable_boot_cpu_mm,
   setup_fixmap, asm_puts, asm_putn  as they will be used externally.
 - Removed early_puts() function definition when EARLY_PRINTK is not
   enabled.

Also move the assembly macros shared by head.S and mmu/head.S to
macros.h.

This is based on 6734327 ("xen/arm64: Split and move MMU-specific head.S to mmu/head.S").

Signed-off-by: Ayan Kumar Halder <[email protected]>
Reviewed-by: Michal Orzel <[email protected]>