bug#24703: Store references in 8-byte chunks in compiled code

[Mark H Weaver <mhw@netris.org>]

The 8-byte chunks may appear out of order.  For example, in
'bash-static' we have this reference to:

  /gnu/store/7z3hpynjsbidxkq78xi5qi6lbcm8ndhp-glibc-intermediate-2.23

where the chunks are found in the following order:

1_/gnu/sto
2_________________ynjsbidx
3_________re/7z3hp
4_________________________________i6lbcm8n
5_________________________kq78xi5q
6_________________________________________dhp-glib
7_________________________________________________c-interm
8_________________________________________________________ediate-2
9_________________________________________________________________.23/lib/

Here's an excerpt of "objdump -d" output, annotated to show the 8-byte
constant as a string, and its position within the larger string.

======== /gnu/store/0010wvgs40kdq8chzsh403qm7la9jxq7-bash-static-4.3.42/bin/bash
  46b2c3:	48 b9 2f 67 6e 75 2f 	movabs $0x6f74732f756e672f,%rcx ; "/gnu/sto" (1)
  46b2ca:	73 74 6f 
  46b2cd:	48 01 d8             	add    %rbx,%rax
  46b2d0:	48 bb 79 6e 6a 73 62 	movabs $0x78646962736a6e79,%rbx ; "ynjsbidx" (3)
  46b2d7:	69 64 78 
  46b2da:	48 89 48 01          	mov    %rcx,0x1(%rax)
  46b2de:	48 b9 72 65 2f 37 7a 	movabs $0x7068337a372f6572,%rcx ; "re/7z3hp" (2)
  46b2e5:	33 68 70 
  46b2e8:	48 89 58 11          	mov    %rbx,0x11(%rax)
  46b2ec:	48 89 48 09          	mov    %rcx,0x9(%rax)
  46b2f0:	48 bb 69 36 6c 62 63 	movabs $0x6e386d63626c3669,%rbx ; "i6lbcm8n" (5)
  46b2f7:	6d 38 6e 
  46b2fa:	48 b9 6b 71 37 38 78 	movabs $0x713569783837716b,%rcx ; "kq78xi5q" (4)
  46b301:	69 35 71 
  46b304:	48 89 48 19          	mov    %rcx,0x19(%rax)
  46b308:	48 89 58 21          	mov    %rbx,0x21(%rax)
  46b30c:	48 b9 64 68 70 2d 67 	movabs $0x62696c672d706864,%rcx ; "dhp-glib" (6)
  46b313:	6c 69 62 
  46b316:	48 bb 63 2d 69 6e 74 	movabs $0x6d7265746e692d63,%rbx ; "c-interm" (7)
  46b31d:	65 72 6d 
  46b320:	48 89 48 29          	mov    %rcx,0x29(%rax)
  46b324:	ba 76 00 00 00       	mov    $0x76,%edx
  46b329:	48 89 58 31          	mov    %rbx,0x31(%rax)
  46b32d:	48 b9 65 64 69 61 74 	movabs $0x322d657461696465,%rcx ; "ediate-2" (8)
  46b334:	65 2d 32 
  46b337:	48 bb 2e 32 33 2f 6c 	movabs $0x2f62696c2f33322e,%rbx ; ".23/lib/" (9)
  46b33e:	69 62 2f 
  46b341:	48 89 58 41          	mov    %rbx,0x41(%rax)
  46b345:	31 f6                	xor    %esi,%esi
  46b347:	31 ff                	xor    %edi,%edi
  46b349:	c6 00 3a             	movb   $0x3a,(%rax)
  46b34c:	48 89 48 39          	mov    %rcx,0x39(%rax)
  46b350:	c7 40 49 67 63 6f 6e 	movl   $0x6e6f6367,0x49(%rax)
  46b357:	66 89 50 4d          	mov    %dx,0x4d(%rax)