Prior to this change, we would unconditionally emit any system include path/framework
path as `-iwithsysroot`/`-iframeworkwithsysroot` if the sysroot was
set which can lead to unexpected build failures. Now, calls to
`b.addSystemIncludePath` and `b.addFrameworkPath` will always emit
search paths as `-isystem`/`-iframework`. As a result, it is now up to
the user to correctly concat the search paths with the sysroot when
and where desired.

If there is a need for emitting `-iwithsysroot`/`-iframeworkwithsysroot`
I would advise adding explicit hooks such as `addSystemIncludePathWithSysroot`
and `addFrameworkPathWithSysroot`.