Allocating an arbitrarily-sized array of tbs results in either
(a) a lot of memory wasted or (b) unnecessary flushes of the code
cache when we run out of TB structs in the array.

An obvious solution would be to just malloc a TB struct when needed,
and keep the TB array as an array of pointers (recall that tb_find_pc()
needs the TB array to run in O(log n)).

Perhaps a better solution, which is implemented in this patch, is to
allocate TB's right before the translated code they describe. This
results in some memory waste due to padding to have code and TBs in
separate cache lines--for instance, I measured 4.7% of padding in the
used portion of code_gen_buffer when booting aarch64 Linux on a
host with 64-byte cache lines. However, it can allow for optimizations
in some host architectures, since TCG backends could safely assume that
the TB and the corresponding translated code are very close to each
other in memory. See this message by rth for a detailed explanation:

  https://lists.gnu.org/archive/html/qemu-devel/2017-03/msg05172.html
  Subject: Re: GSoC 2017 Proposal: TCG performance enhancements
  Message-ID: <1e67644b-4b30-887e-d329-1848e94c9484@twiddle.net>
Suggested-by: NRichard Henderson <rth@twiddle.net>
Reviewed-by: NPranith Kumar <bobby.prani@gmail.com>
Signed-off-by: NEmilio G. Cota <cota@braap.org>
Message-Id: <1496790745-314-3-git-send-email-cota@braap.org>
[rth: Simplify the arithmetic in tcg_tb_alloc]
Signed-off-by: NRichard Henderson <rth@twiddle.net>