have xpfpa use no-op macros for ARM-linux compiles; doubles s/b OK

While arm-linux C libs have floating point control similar to x86,
they may be missing some of the constants used to compose control
words that xpfpa.h expects to be there. So don't let ARM compiles
use the macros intended for x86. Research suggests doubles in ARM
environments will be the expected 64 bit (53 bit precision) doubles
we need, without any extra fiddling.
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