Kernel: Fix signal generation

We need to have interrupts enabled when signal kills the process as
process does mutex locking. Also signals are now only checked when
returning to userspace in the same place where userspace segments are
loaded.

kernel/arch/x86_64/interrupts.S

@@ -1,12 +1,4 @@
-.macro swapgs_if_necessary, n
-	testb $3, \n(%rsp)
-	jz 1f; jnp 1f
-	swapgs
-1:
-.endm
-
-.macro pushaq, n
-	swapgs_if_necessary \n
+.macro intr_header, n
 	pushq %rax
 	pushq %rcx
 	pushq %rdx
@@ -22,10 +14,18 @@
 	pushq %r13
 	pushq %r14
 	pushq %r15
+	testb $3, \n+15*8(%rsp)
+	jz 1f
+	swapgs
+1:	cld
 .endm
 
-.macro popaq, n
-	popq %r15
+.macro intr_footer, n
+	testb $3, \n+15*8(%rsp)
+	jz 1f
+	call cpp_check_signal
+	swapgs
+1:	popq %r15
 	popq %r14
 	popq %r13
 	popq %r12
@@ -40,12 +40,10 @@
 	popq %rdx
 	popq %rcx
 	popq %rax
-	swapgs_if_necessary \n
 .endm
 
 isr_stub:
-	pushaq 24
-	cld
+	intr_header 24
 	movq %cr0, %rax; pushq %rax
 	movq %cr2, %rax; pushq %rax
 	movq %cr3, %rax; pushq %rax
@@ -58,33 +56,33 @@ isr_stub:
 	call cpp_isr_handler
 	addq $32, %rsp
 
-	popaq 24
+	intr_footer 24
 	addq $16, %rsp
 	iretq
 
 irq_stub:
-	pushaq 24
-	cld
+	intr_header 24
+	xorq %rbp, %rbp
 	movq 120(%rsp), %rdi	# irq number
 	call cpp_irq_handler
-	popaq 24
+	intr_footer 24
 	addq $16, %rsp
 	iretq
 
 .global asm_ipi_handler
 asm_ipi_handler:
-	pushaq 8
-	cld
+	intr_header 8
+	xorq %rbp, %rbp
 	call cpp_ipi_handler
-	popaq 8
+	intr_footer 8
 	iretq
 
 .global asm_timer_handler
 asm_timer_handler:
-	pushaq 8
-	cld
+	intr_header 8
+	xorq %rbp, %rbp
 	call cpp_timer_handler
-	popaq 8
+	intr_footer 8
 	iretq
 
 .macro isr n