64-bit ARM MMAP_MIN_ADDR recommendation should be based on whether CONFIG_COMPAT is enabled

[thestinger]

This is meant to be 32768 not only on 32-bit ARM but also 64-bit ARM when 32-bit process compatibility is enabled. It should check for 32-bit compatibility and make the recommendation based on it. There's already a recommendation to disable 32-bit support for attack surface reduction and being able to raise MMAP_MIN_ADDR one a tiny, largely insignificant side benefit.

Here's a relevant snippet from security/Kconfig:

config LSM_MMAP_MIN_ADDR
	int "Low address space for LSM to protect from user allocation"
	depends on SECURITY && SECURITY_SELINUX
	default 32768 if ARM || (ARM64 && COMPAT)
	default 65536