Long ago, this code was added to disable -O2 optimizations and
presumably remove -mcpu flags to avoid miscompilation. Somewhere along
the line it stopped working and code is being compiled with -O2 by
default and proably with -mcpu flags if set. The only place this seems
to be used is when checking if the compiler works early in configure.

Discovered on CheriBSD where removing -m* removed created an invalid
command line with two -Xclang flags in a row.

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