vm: remove stack handling for signals

. moved to the kernel as the handling was only
	  reading it; the kernel may as well write it too

kernel/arch/i386/arch_system.c

@@ -545,6 +545,8 @@ void fpu_sigcontext(struct proc *pr, struct sigframe *fr, struct sigcontext *sc)
 	}
 }
 
+reg_t arch_get_sp(struct proc *p) { return p->p_reg.sp; }
+
 #if !CONFIG_OXPCIE
 static void ser_init(void)
 {

kernel/proto.h

@@ -206,6 +206,7 @@ void arch_do_syscall(struct proc *proc);
 int arch_phys_map(int index, phys_bytes *addr, phys_bytes *len, int
 	*flags);
 int arch_phys_map_reply(int index, vir_bytes addr);
+reg_t arch_get_sp(struct proc *p);
 int arch_enable_paging(struct proc * caller);
 int vm_check_range(struct proc *caller,
        struct proc *target, vir_bytes vir_addr, size_t bytes);

kernel/system/do_sigsend.c

@@ -38,6 +38,7 @@ int do_sigsend(struct proc * caller, message * m_ptr)
 	return r;
 
   /* Compute the user stack pointer where sigcontext will be stored. */
+  smsg.sm_stkptr = arch_get_sp(rp);
   scp = (struct sigcontext *) smsg.sm_stkptr - 1;
 
   /* Copy the registers to the sigcontext structure. */