testsuite/libffi.call/many_win32.c: Avoid spurious failure due to excess precision

The test case testsuite/libffi.call/many_win32.c can spuriously fail due
to excess floating-point precision.  Instrumenting it with some printf
calls shows differences well above FLT_EPSILON.  (Note when
instrumenting it that multiple computations of the difference, such as
one in a print and another in the conditional, may produce different
results.)

Rather than complicating the test suite with architecture-specific flags
to avoid excess precision, just simplify the floating-point computation
to avoid a dependency on potential excess precision.