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>