doc: Clarify x86_64 build host limitation

Add x86_64 host requirement to the Build Host Environment Setup section.

Add a related limitation regarding the x86_64 host requirement.

Issue-Id: SCM-5314
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: I8bcd76e9ef5a989437eaaee4ae8341002bbf3aca