cmake: use standard cross compile toolchain file

remove CROSS_COMPILE_ARM64 option and update README.md to instruct on
using the standard cmake way of cross compiling via a toolchain file.

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
3 jobs for cmake-fixes in 7 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
  X86 64
passed #3167770
kvm
x86_64

00:03:49

 
  Aarch64
passed #3167772
aarch64
aarch64

00:03:05

passed #3167771
kvm
perfetto

00:03:12