plat/common/x86: Sanitize the ECTX slot on syscall entry

plat/common/x86: Sanitize the ECTX slot on syscall entry

Commit c716bcca4822 ("{lib,arch,plat}: Redo syscall ctx's and swapgs logic"),
following a rework of architecture specific contexts and syscall entries,
by mistake removed the ECTX sanitization at the beginning of system calls.
This can result in #GP on x86 if the XSAVE header happens to be dirty.
Thus, bring this sanitization back.

Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Radu Nichita <radunichita99@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #1444