Kernel: Fix the gettid syscall

syscall_handler was not actually updating the value in regs->eax, so the
gettid() was always returning 85: the value of regs->eax was not
actually updated, and it remained the one from Userland (the value of
SC_gettid).

The syscall_handler was modified to actually get a pointer to
RegisterState, so any changes to it will actually be saved.

NOTE: This was actually more of a compiler optimization:
On the SC_gettid flow, we saved in regs.eax the return value of
sys$gettid(), but the compiler discarded it, since it followed a return.
On a normal flow, the value of regs.eax was reused in
tracer->did_syscall, so the compiler actually updated the value.

Kernel/Syscall.cpp

@@ -33,7 +33,7 @@
 
 namespace Kernel {
 
-extern "C" void syscall_handler(RegisterState);
+extern "C" void syscall_handler(RegisterState&);
 extern "C" void syscall_asm_entry();
 
 asm(
@@ -52,8 +52,9 @@ asm(
     "    cld\n"
     "    xor %esi, %esi\n"
     "    xor %edi, %edi\n"
+    "    push %esp\n"
     "    call syscall_handler\n"
-    "    add $0x4, %esp\n"
+    "    add $0x8, %esp\n"
     "    popl %gs\n"
     "    popl %fs\n"
     "    popl %es\n"
@@ -121,7 +122,7 @@ int handle(RegisterState& regs, u32 function, u32 arg1, u32 arg2, u32 arg3)
 
 }
 
-void syscall_handler(RegisterState regs)
+void syscall_handler(RegisterState& regs)
 {
     // Special handling of the "gettid" syscall since it's extremely hot.
     // FIXME: Remove this hack once userspace locks stop calling it so damn much.