Function init_syscall 

pub fn init_syscall()

Initialize SYSCALL/SYSRET support.

Configures the following MSRs:

• EFER: Enable SYSCALL/SYSRET extensions
• LSTAR: Set syscall entry point to syscall_entry
• STAR: Set segment selectors for SYSCALL (kernel) and SYSRET (user)
• SFMASK: Mask IF flag so syscall entry runs with interrupts disabled
• KernelGsBase: Point to PerCpuData for swapgs in syscall_entry

Must be called after gdt::init() and before any user-mode transitions.