iOS: Fix slow debugger startup on devices

We need to pass the path to the device symbols, that Xcode downloaded
for the device's OS version, as the sysroot to the debugger. Otherwise
debugger startup is very slow.

We already tried to do that, but it looks like, depending on the
devices, this path can contain an architecture specific part, e.g.
"iOS DeviceSupport/13.5.1 (17F80) arm64e" instead of just "iOS
DeviceSupport/13.5.1 (17F80)". It can still be just the latter, so we
get the devices architecture information, try the architecture specific
directory first, and fall back to the architecture agnostic name as
before if the former doesn't exist.

Fixes: QTCREATORBUG-21682
Change-Id: I2efdbfda0282f1cf0f8d10bd4e5217a298027fcf
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/ios/iosdevice.cpp

@@ -171,6 +171,11 @@ QString IosDevice::osVersion() const
     return m_extraInfo.value(QLatin1String("osVersion"));
 }
 
+QString IosDevice::cpuArchitecture() const
+{
+    return m_extraInfo.value("cpuArchitecture");
+}
+
 Utils::Port IosDevice::nextPort() const
 {
     // use qrand instead?