[Pal/Linux] Fix _DkThreadExit call in thread_start

The old code had one indirection too much. This fixes the case that the
thread function returns (instead of calling DkThreadExit). Before this
triggered a SIGSEGV.

Pal/src/host/Linux/clone-x86_64.S

@@ -108,7 +108,7 @@ thread_start:
 	/* movl $SYS_ify(exit), %eax */
 	/* syscall */
 	/* Instead of syscall exit, let's call _DkThreadExit */
-	call *_DkThreadExit(%rip)
+	call _DkThreadExit
 	cfi_endproc
 
 	cfi_startproc