Compiler feature detection failure if multilib toolchain needs platform code generation flags

Qt Creator 4.11 is showing a new error opening projects using our wineg++ kit with a yocto cross-compiling gcc toolchain underneath:

Compiler feature detection failure!
The command "/opt/YukonAtom/sysroots/core2-32-wrs-linux/usr/bin/winegcc -print-search-dirs" terminated with exit code 2.
winegcc: i686-wrs-linux-gcc failed

Compiler feature detection failure!
The command "/opt/YukonAtom/sysroots/core2-32-wrs-linux/usr/bin/wineg++ -dumpmachine" terminated with exit code 2.
winegcc: i686-wrs-linux-g++ failed

Compiler feature detection failure!
The command "/opt/YukonAtom/sysroots/core2-32-wrs-linux/usr/bin/wineg++ -print-search-dirs" terminated with exit code 2.
winegcc: i686-wrs-linux-g++ failed

Thee problem here is that winegcc/wineg++ are compiler wrappers, and need to find the underlying target gcc/g++. To do that, it needs to be provided with flags like -B/opt/TargetSDK/sysroots/x86_64-wrlinuxsdk-linux/usr/bin/i686-wrs-linux --target=i686-wrs-linux, and potentially --sysroot=/opt/TargetSDK/sysroots/i686-wrs-linux.

I've used sdktool addTC ... to set up this compiler and kit in Qt Creator, and provided the needed flags in ProjectExplorer.GccToolChain.PlatformCodeGenFlags. gccPrepareArguments has a whitelist of which codegen flags to pass through when probing, and functions like GccToolChain::builtInHeaderPaths are using this and succeeding. But the guessGccAbi, gccVersion, and gccInstallDir do not, so they are failing.

Comparing this code in 4.10.1, 4.11.0, and master, it does not appear that the actual failure is anything new: it just seems to have become more visible, because probing failures are now written to Core::MessageManager instead of qWarning() (this changed in b4022f9).