README: Update build instructions for macOS

Pre-built cross-compilers for x86 are available in homebrew and can be
used to build the tests.

Cc: Cameron Esfahani <dirty@apple.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200901085056.33391-8-r.bolshakov@yadro.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>