ARC: fp: (micro)optimize FPU_STATUS read by eliding FWE bit clearing

Any FPU_STATUS write needs setting the FWE bit (31) whcih just provides
a "control signal" to enable explicit write (vs. the side-effect of FPU
instructions).  However this bit is RAZ and write-only, thus effectively
never stored in FPU_STATUS register. Thus when reading the register
there is no need to clear it. This shaves off a BCLR instruction from
the fe*exceptino family of functions and while no big deal still makes
sense to do.

This came up when debugging a race in math/test-fenv-tls [1]

[1]: https://github.com/foss-for-synopsys-dwc-arc-processors/linux/issues/54



Signed-off-by: Vineet Gupta <vgupta@synopsys.com>