perf tests: Fix flaky test 'Object code reading'

perf tests: Fix flaky test 'Object code reading'

This test occasionally fails on aarch64 when a sample is taken in
free@plt and it fails with "Bytes read differ from those read by
objdump".

This is because that symbol is near a section boundary in the elf file.
Despite the -z option to always output zeros, objdump uses
bfd_map_over_sections() to iterate through the elf file so it doesn't
see outside of the sections where these zeros are and can't print them.

For example this boundary proceeds free@plt in libc with a gap of 48
bytes between .plt and .text:

  objdump -d -z --start-address=0x23cc8 --stop-address=0x23d08 libc-2.30.so

  libc-2.30.so:     file format elf64-littleaarch64

  Disassembly of section .plt:

  0000000000023cc8 <*ABS*+0x7fd00@plt+0x8>:
     23cc8:	91018210 	add	x16, x16, #0x60
     23ccc:	d61f0220 	br	x17

  Disassembly of section .text:

  0000000000023d00 <abort@@GLIBC_2.17-0x98>:
     23d00:	a9bf7bfd 	stp	x29, x30, [sp, #-16]!
     23d04:	910003fd 	mov	x29, sp

Taking a sample in free@plt is very rare because it is so small, but the
test can be forced to fail almost every time on any platform by linking
the test with a shared library that has a single empty function and
calling it in a loop.

The fix is to zero the buffers so that when there is a jump in the
addresses output by objdump, zeros are already filled in between.

Signed-off-by: James Clark <james.clark@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lore.kernel.org/lkml/20210906152238.3415467-1-james.clark@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>