Fix parsing /proc/pid/maps dump in CPU profile data file

When trying to use pprof on my machine, the symbols of my program were
not being recognized.

It turned out that pprof, when calculating the offset of the text list
of mapped objects (the last section of the CPU profile data file), was
assuming that the slot size was always 4 bytes, even on 64-bit machines.

This led to ParseLibraries() reading a lot of garbage data at the
beginning of the map, and consequently the regex was failing to match on
the first line of the real (non-garbage) map.
issue-654: [pprof] handle split text segments

This applies patch by user simonb.

Quoting:

Relocation packing splits a single executable load segment into two.  Before:

  LOAD           0x000000 0x00000000 0x00000000 0x2034d28 0x2034d28 R E 0x1000
  LOAD           0x2035888 0x02036888 0x02036888 0x182d38 0x1a67d0 RW  0x1000

After:
  LOAD           0x000000 0x00000000 0x00000000 0x14648 0x14648 R E 0x1000
  LOAD           0x014648 0x0020c648 0x0020c648 0x1e286e0 0x1e286e0 R E 0x1000
  ...
  LOAD           0x1e3d888 0x02036888 0x02036888 0x182d38 0x1a67d0 RW  0x1000

The .text section is in the second LOAD, and this is not at
offset/address zero.  The result is that this library shows up in
/proc/self/maps as multiple executable entries, for example (note:
this trace is not from the library dissected above, but rather from an
earlier version of it):

  73b0c000-73b21000 r-xp 00000000 b3:19 786460 /data/.../libchrome.2160.0.so
  73b21000-73d12000 ---p 00000000 00:00 0
  73d12000-75a90000 r-xp 00014000 b3:19 786460 /data/.../libchrome.2160.0.so
  75a90000-75c0d000 rw-p 01d91000 b3:19 786460 /data/.../libchrome.2160.0.so

When parsing this, pprof needs to merge the two r-xp entries above
into a single entry, otherwise the addresses it prints are incorrect.

The following fix against 2.2.1 was sufficient to make pprof --text
print the correct output.  Untested with other pprof options.
issue-493: correctly detect __ARM_ARCH_6ZK__ for MemoryBarrier

Which should fix issue reported by user pedronavf