os_set_sigcontext_fpu_modes missing on x86
The function %set-sigcontext-floating-point-modes
calls the C function os_set_sigcontext_fpu_modes
which is missing.
We don't seem to use %set-sigcontext-floating-point-modes
though. The ppc port has such a function, but the sparc port does not.
Delete the function or add the C implementation of it.