Only use -fp-exception-behavior=maytrap on x86, for now

After 3b00222f156d, it turns out that clang only supports strict
floating point semantics for SystemZ and x86 at the moment, while for
other architectures it is still experimental.

Therefore, only use -fp-exception-behavior=maytrap on x86 for now,
otherwise this option results in "error: overriding currently
unsupported use of floating point exceptions on this target
[-Werror,-Wunsupported-floating-point-opt]" on other architectures.

Fixes: 3b00222f156d
PR: 254911
MFC after: 1 week