docs: Add note about AArch64 build environment

This patch updates the documentation to mention support
for AArch64 build host and also clarifies the Ubuntu version
requirement for the build host.

Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: I2e5f8171ce9ea37c468e3767341949af30eb2aef
1 file changed