Call x86-64 __mcount_internal/__sigjmp_save directly

Since __mcount_internal and __sigjmp_save are internal to x86-64 libc.so:

3532: 0000000000104530   289 FUNC    LOCAL  DEFAULT   13 __mcount_internal
3391: 0000000000034170    38 FUNC    LOCAL  DEFAULT   13 __sigjmp_save

they can be called directly without PLT.

	* sysdeps/x86_64/_mcount.S (C_LABEL(_mcount)): Call
	__mcount_internal directly.
	(C_LABEL(__fentry__)): Likewise.
	* sysdeps/x86_64/setjmp.S __sigsetjmp): Call __sigjmp_save
	directly.