i386: Don't unnecessarily save and restore EAX, ECX and EDX [BZ# 25262]
    
    On i386, since EAX, ECX and EDX are caller-saved, there are no need
    to save and restore EAX, ECX and EDX in getcontext, setcontext and
    swapcontext.  They just need to clear EAX on success.  The extra
    scratch registers are needed to enable CET.
    
    Tested on i386.
    
    Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>