ldso: exit if zalloc can't alloc memory

_dl_zalloc callers don't check for allocaton failure. It kind of makes
sense since such early allocations are unlikely to fail, and even if
they do, ldso would segv anyways thus bringing the issue to attention.

However there's a gcc nuance which led to this patch.

It seems gcc at -O2 (for DODEBUG build), does additional coge gen
analysis for pointer dereference from erroneous paths and it "isolates"
such code paths with a intrinsic abort/trap etc.

The ldso code fragment which was triggering this:

| add_ldso(struct dyn_elf *rpnt)
|    if (rpnt)
|        ...
|    else
|       rpnt = _dl_zalloc()
|
|    rpnt->dyn = tpnt  <---- potential NULL pointer deref

ARC gcc currently generates an abort() call which doesn't exist in ldso,
causing link errors (with a newer vrsion of binutils).

ARM gcc 6.2.1 behaves similarly, altough instead of abort, it generates
a trap inducing UDF instruction

|    367c:	ebfffb79   bl	2468 <_dl_malloc>
|    3680:	e51b2068   ldr	r2, [fp, #-104]	; 0xffffff98
|    3684:	e3500000   cmp	r0, #0
|    3688:	0a000006   beq	36a8 <_dl_add_elf_hash_table+0x280>
| ...
|    36a8:	e5862000   str	r2, [r6]
|    36ac:	e7f000f0   udf	#

So add an explict dl_exit() in dl_zalloc error case to beat the
compiler.

Note that this error propagagtion analysis stops if the function in
consideration (_dl_zalloc) is NOT inlined. Hence the reason it only
shows up for DODEBUG builds which builds ldso at -O2 which is more
aggressive about inlining.

If this patch is not considered worth applying then the workaround
suggested by Claudiu is to to build ldso with
-fno-isolate-erroneous-paths-dereference

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

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