Disable clang's -fintegrated-cc1 stage by default

In bug 246630, it was found that part of the rescue binary could be
compiled to very slightly different (but still equivalent) machine code,
depending on the number of simultaneous make jobs (via the -j option).

This turned out to be caused by the upstream change that made clang's
first stage compiler (i.e. the -cc1 stage) run as part of the initial
clang process invocation, instead of forking and exec'ing a new clang
process.

We are currently investigating the root cause for the difference in
output, but while that is ongoing, disable the integrated cc1 stage for
now to work around it. You can always turn it on explicitly by using the
-fintegrated-cc1 option, or turn it off with -fno-integrated-cc1.

Direct commit to stable/{11,12}, so this can hopefully end up in the
upcoming 11.4-RELEASE.

Reported by: Fabian Keil <fk@fabiankeil.de>
PR: 246630