[AVR] Fix private label prefix

This is a small pet peeve from me. This change makes sure the AVR backend uses
the correct private label prefix (.L) so that private labels are hidden in
avr-objdump.

Example code:

    define i8 @foo(i1 %cond) {
      br i1 %cond, label %then, label %else
    then:
      ret i8 3
    else:
      ret i8 5
    }

When compiling this:
  llc -march=avr -filetype=obj -o test.o test.ll
and then dumping it:
  avr-objdump -d test.o
You would previously get an ugly temporary label:

    00000000 <foo>:
       0:        81 70       andi       r24, 0x01    ; 1
       2:        80 30       cpi        r24, 0x00    ; 0
       4:        f9 f3       breq       .-2          ; 0x4 <foo+0x4>
       6:        83 e0       ldi        r24, 0x03    ; 3
       8:        08 95       ret

    0000000a <LBB0_2>:
       a:        85 e0       ldi        r24, 0x05    ; 5
       c:        08 95       ret

This patch fixes that, the output is now:

    00000000 <foo>:
       0:        81 70       andi       r24, 0x01    ; 1
       2:        80 30       cpi        r24, 0x00    ; 0
       4:        01 f0       breq       .+0          ; 0x6 <foo+0x6>
       6:        83 e0       ldi        r24, 0x03    ; 3
       8:        08 95       ret
       a:        85 e0       ldi        r24, 0x05    ; 5
       c:        08 95       ret

Note that as you can see the breq operand is different. However it is
still the same after linking:

       4:        11 f0       breq       .+4

Differential Revision: https://reviews.llvm.org/D75124