Make QT_CPU_FEATURES a map of features based on architecture

For now there's only one architecture per host/target, but
this will change once we start detecting the CPU features
for both device and simulator on iOS.

For convenience we set QT_CPU_FEATURES to the resolved
value of the current architecture, so that simd.prf still
can use QT_CPU_FEATURES directly.

Change-Id: I28e8b339a5c30a630e276165254dba09a3da6940
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Scope QT_COMPILER_SUPPORTS_xxx by processor architecture in qconfig.h

Change-Id: Ie306b7b26e7dde641177ad54daf4365085fe9dce
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>