arm: Remove wrong ldr from _dl_start_user (BZ 31339)

The commit 49d877a80b (arm: Remove
_dl_skip_args usage) removed the _SKIP_ARGS literal, which was
previously loader to r4 on loader _start.  However, the cleanup did not
remove the following 'ldr r4, [sl, r4]' on _dl_start_user, used to check
to skip the arguments after ld self-relocations.

In my testing, the kernel initially set r4 to 0, which makes the
ldr instruction just read the _GLOBAL_OFFSET_TABLE_.  However, since r4
is a callee-saved register; a different runtime might not zero
initialize it and thus trigger an invalid memory access.

Checked on arm-linux-gnu.

Reported-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
(cherry picked from commit 1e25112dc0)

sysdeps/arm/dl-machine.h

@@ -139,7 +139,6 @@ _start:\n\
 _dl_start_user:\n\
 	adr	r6, .L_GET_GOT\n\
 	add	sl, sl, r6\n\
-	ldr	r4, [sl, r4]\n\
 	@ save the entry point in another register\n\
 	mov	r6, r0\n\
 	@ get the original arg count\n\