refactor: remove #if x86 guard from scheduler fork_child_trampoline

refactor: remove #if x86 guard from scheduler fork_child_trampoline

Add hal_usermode_enter_regs() to the HAL usermode interface so
fork_child_trampoline() can call it generically without #if x86.

- Add declaration to include/hal/usermode.h
- Implement in src/hal/x86/usermode.c (calls x86_enter_usermode_regs)
- Add stubs in ARM, RISC-V, MIPS HAL usermode.c
- scheduler.c now uses hal_usermode_enter_regs() with no guards

Passes: make, cppcheck, QEMU smoke test.