x86: move i386 memory setup code to e820_32.c

The x86_64 code has centralized the memory setup code in
e820_64.c. This patch copies that approach to i386:

- early_param("mem", ...) parsing is moved from
setup_32.c to e820_32.c.

- setup_memory_map() and finish_e820_parsing() are
factored out from setup_arch(), and declarations
are added to e820_32.h.

- print_memory_map() is made static and removed from
e820_32.h.

- user_defined_memmap is marked as __initdata.

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>