Linux supports the stdcall calling convention, either via functions
explicitly declared with the stdcall attribute, or via code compiled
with -mrtd which effectively makes stdcall the default.
This introduces FFI_STDCALL, FFI_THISCALL, and FFI_FASTCALL on
non-Windows x86-32 platforms, as non-default calling conventions.
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.
