refactor: abstract x86 register accesses in syscall dispatcher via sc_* macros

refactor: abstract x86 register accesses in syscall dispatcher via sc_* macros

- include/arch/x86/arch_syscall.h: define sc_num/sc_arg0..4/sc_ret/
  sc_ip/sc_usp macros mapping to x86 INT 0x80 ABI registers
  (eax/ebx/ecx/edx/esi/edi/eip/useresp)
- include/arch_syscall.h: generic dispatch header with non-x86 stubs
- src/kernel/syscall.c: replace all ~200 direct regs->eax/ebx/ecx/
  edx/esi/edi/eip/useresp accesses with arch-agnostic sc_* macros
  across syscall_handler, posix_ext_syscall_dispatch, and
  socket_syscall_dispatch

syscall.c now contains zero x86-specific register names. To port to
ARM, only arch/arm/arch_syscall.h needs to map sc_* to ARM registers
(r7/r0-r4/pc/sp).