x86/syscall: Split IDT syscall setup code into idt_syscall_init()

Because FRED uses the ring 3 FRED entrypoint for SYSCALL and SYSENTER and
ERETU is the only legit instruction to return to ring 3, there is NO need
to setup SYSCALL and SYSENTER MSRs for FRED, except the IA32_STAR MSR.

Split IDT syscall setup code into idt_syscall_init() to make it easy to
skip syscall setup code when FRED is enabled.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Xin Li <xin3.li@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Shan Kang <shan.kang@intel.com>
Link: https://lore.kernel.org/r/20231205105030.8698-34-xin3.li@intel.com