[Pal/{Linux, Linux-SGX}] Remove duplicated labels in clone-x86_64.S

ENTRY() macro already defines a label, so the labels don't need to be
defined again.

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

@@ -91,7 +91,6 @@ ENTRY (clone)
 	ret
 
 ENTRY (thread_start)
-thread_start:
 	cfi_startproc
 	/* Clearing frame pointer is insufficient, use CFI.  */
 	cfi_undefined (rip)

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

@@ -91,7 +91,6 @@ ENTRY (clone)
 	ret
 
 ENTRY (thread_start)
-thread_start:
 	cfi_startproc
 	/* Clearing frame pointer is insufficient, use CFI.  */
 	cfi_undefined (rip)