This moves the x86 syscall table into thread_info to allow for per-thread syscall tables. Native 32-bit, 64-bit, and 32-bit emulation are handled. The feature is enabled with CONFIG_ALT_SYSCALL. Based on proof-of-concept from Will Drewry. BUG=brillo:384 TEST=gizmo build & boot, manual testing Suggested-by: Will Drewry <wad@chromium.org> Signed-off-by: Kees Cook <keescook@chromium.org> Change-Id: I2adb4742e284e8555621ccd78e487d8e4be73f6c Reviewed-on: https://chromium-review.googlesource.com/253230 Reviewed-by: Aaron Durbin <adurbin@chromium.org>