In preparation for https://gitlab.winehq.org/wine/wine/-/merge_requests/1324.
This also begins preparation for a slightly different route than what the MR currently takes, with syscall flags eventually stored in the CounterTable rather than overusing syscall number unused bits.
To do that we're checking the syscall number and loading the syscall table (keeping it in %rbx/%ebx) earlier. This assumes that %rbx isn't modified in between, for instance by the eventual `SYS_arch_prctl` syscall, but I believe it is the case?
-- v2: ntdll: Check SYSCALL_HAVE_WRFSGSBASE syscall flag only for wrfsbase. ntdll: Swap %eax and %edx registers in the i386 syscall dispatcher. ntdll: Check syscall table and syscall number before saving FPU. ntdll: Use named labels for jumps in the syscall dispatcher.