proc: fix gs/fs-related bug in exception handler

1. %rip -> %rsp (misspelt a register name, damn)
2. Rename swapgs_if_needed to iret_swapgs_if_needed for clarification
3. Add iret_swapgs_if_needed before iret from exception handler

include/arch/amd64/asm/asm_cpu.h

@@ -11,9 +11,9 @@
     (__amd64_cpu + off)(%rip)
 #endif
 
-// To be used in interrupt handlers
+// To be used in interrupt handlers or anything with iret-like stack layout
 // DIRECTLY AFTER ENTRY without prior stack modification
-.macro swapgs_if_needed
+.macro iret_swapgs_if_needed
     cmpq $0x08, 0x08(%rsp)
     je 1f
     swapgs

include/arch/amd64/asm/asm_irq.h

@@ -13,7 +13,7 @@
 .global amd64_irq\n
 amd64_irq\n:
     cli
-    swapgs_if_needed
+    iret_swapgs_if_needed
 
     pushq %r11
     pushq %r10
@@ -39,7 +39,7 @@ amd64_irq\n:
     popq %r10
     popq %r11
 
-    swapgs_if_needed
+    iret_swapgs_if_needed
     iretq
 .endm