x86_64: Suppress false positive valgrind error

x86_64: Suppress false positive valgrind error

When strcmp-avx2.S is used as the default, elf/tst-valgrind-smoke fails
with

==1272761== Conditional jump or move depends on uninitialised value(s)
==1272761==    at 0x4022C98: strcmp (strcmp-avx2.S:462)
==1272761==    by 0x400B05B: _dl_name_match_p (dl-misc.c:75)
==1272761==    by 0x40085F3: _dl_map_object (dl-load.c:1966)
==1272761==    by 0x401AEA4: map_doit (rtld.c:644)
==1272761==    by 0x4001488: _dl_catch_exception (dl-catch.c:237)
==1272761==    by 0x40015AE: _dl_catch_error (dl-catch.c:256)
==1272761==    by 0x401B38F: do_preload (rtld.c:816)
==1272761==    by 0x401C116: handle_preload_list (rtld.c:892)
==1272761==    by 0x401EDF5: dl_main (rtld.c:1842)
==1272761==    by 0x401A79E: _dl_sysdep_start (dl-sysdep.c:140)
==1272761==    by 0x401BEEE: _dl_start_final (rtld.c:494)
==1272761==    by 0x401BEEE: _dl_start (rtld.c:581)
==1272761==    by 0x401AD87: ??? (in */elf/ld.so)

The assembly codes are:

   0x0000000004022c80 <+144>:	vmovdqu 0x20(%rdi),%ymm0
   0x0000000004022c85 <+149>:	vpcmpeqb 0x20(%rsi),%ymm0,%ymm1
   0x0000000004022c8a <+154>:	vpcmpeqb %ymm0,%ymm15,%ymm2
   0x0000000004022c8e <+158>:	vpandn %ymm1,%ymm2,%ymm1
   0x0000000004022c92 <+162>:	vpmovmskb %ymm1,%ecx
   0x0000000004022c96 <+166>:	inc    %ecx
=> 0x0000000004022c98 <+168>:	jne    0x4022c32 <strcmp+66>

strcmp-avx2.S has 32-byte vector loads of strings which are shorter than
32 bytes:

(gdb) p (char *) ($rdi + 0x20)
$6 = 0x1ffeffea20 "memcheck-amd64-linux.so"
(gdb) p (char *) ($rsi + 0x20)
$7 = 0x4832640 "core-amd64-linux.so"
(gdb) call (int) strlen ((char *) ($rsi + 0x20))
$8 = 19
(gdb) call (int) strlen ((char *) ($rdi + 0x20))
$9 = 23
(gdb)

It triggers the valgrind error.  The above code is safe since the loads
don't cross the page boundary.  Update tst-valgrind-smoke.sh to accept
an optional suppression file and pass a suppression file to valgrind when
strcmp-avx2.S is the default implementation of strcmp.
Reviewed-by: Sunil K Pandey <skpgkp2@gmail.com>