Add Safe-Linking to fastbins

Safe-Linking is a security mechanism that protects single-linked
lists (such as the fastbins) from being tampered by attackers. The
mechanism makes use of randomness from ASLR (mmap_base), and when
combined with chunk alignment integrity checks, it protects the
pointers from being hijacked by an attacker.

While Safe-Unlinking protects double-linked lists (such as the small
bins), there wasn't any similar protection for attacks against
single-linked lists. This solution protects against 3 common attacks:
  * Partial pointer override: modifies the lower bytes (Little Endian)
  * Full pointer override: hijacks the pointer to an attacker's location
  * Unaligned chunks: pointing the list to an unaligned address

The design assumes an attacker doesn't know where the heap is located,
and uses the ASLR randomness to "sign" the single-linked pointers. We
mark the pointer as P and the location in which it is stored as L, and
the calculation will be:
  * PROTECT(P) := (L >> PAGE_SHIFT) XOR (P)
  * *L = PROTECT(P)

This way, the random bits from the address L (which start at the bits
in the PAGE_SHIFT position), will be merged with the LSB of the stored
protected pointer. This protection layer prevents an attacker from
modifying the pointer into a controlled value.

An additional check that the chunks are MALLOC_ALIGNed adds an
important layer:
  * Attackers can't point to illegal (unaligned) memory addresses
  * Attackers must guess correctly the alignment bits

On standard 32 bit Linux machines, an attacker will directly fail 7
out of 8 times, and on 64 bit machines it will fail 15 out of 16
times.

The proposed solution adds 3-4 asm instructions per malloc()/free()
and therefore has only minor performance implications if it has
any. A similar protection was added to Chromium's version of TCMalloc
in 2013, and according to their documentation the performance overhead
was less than 2%.

Signed-off-by: Eyal Itkin <eyalit@checkpoint.com>

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