x86/mtrr: Move 32-bit code from mtrr.c to legacy.c

x86/mtrr: Move 32-bit code from mtrr.c to legacy.c

There is some code in mtrr.c which is relevant for old 32-bit CPUs
only. Move it to a new source legacy.c.

While modifying mtrr_init_finalize() fix spelling of its name.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20230502120931.20719-9-jgross@suse.com
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>