gitian: add statically built variant of bitcoind/bitcoin-cli for linux build

See #3803 and #3781.

This increases compatibility with older linux distributions.

      2943584 Mar 20 11:43 32/bitcoin-cli
      4376728 Mar 20 11:43 32/bitcoin-cli.static
      7748512 Mar 20 11:43 32/bitcoind
      9140856 Mar 20 11:43 32/bitcoind.static
     12164000 Mar 20 11:43 32/bitcoin-qt
      9740608 Mar 20 11:43 32/test_bitcoin
      8420256 Mar 20 11:43 32/test_bitcoin-qt
      3103728 Mar 20 11:53 64/bitcoin-cli
      4355112 Mar 20 11:53 64/bitcoin-cli.static
      7773680 Mar 20 11:53 64/bitcoind
      8890088 Mar 20 11:53 64/bitcoind.static
     12226032 Mar 20 11:53 64/bitcoin-qt
      9860528 Mar 20 11:53 64/test_bitcoin
      8457776 Mar 20 11:53 64/test_bitcoin-qt

A warning shown during build is:

     warning: Using 'getaddrinfo' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking

~~**Note** I have not explicitly disabled hardening/-pie in this build. This makes that the 32-bit version still has a dependency on dynamic libc.so even though it is statically linked:~~

    $ ldd 32/bitcoind.static
        statically linked
    $ readelf -a 32/bitcoind.static |grep interpreter
          [Requesting program interpreter: /usr/lib/libc.so.1]

~~Not sure how much of an issue this is on true 32-bit systems. It needs to be tested. This is not the case for the 64-bit build:~~

    $ ldd 64/bitcoind.static                                                       
        not a dynamic executable
    $ readelf -a 64/bitcoind.static |grep interpreter
    (nothing)

Edit: v2: -pie now explicitly disabled for 32-bit build as it was unusable otherwise.

Tested on:
- Debian 7.2 64-bit: OK (tested by me)
- Ubuntu 10.04.4 LTS 64-bit: OK (tested by me)
- Ubuntu 10.04.4 LTS 32-bit: OK (tested by me, v2)
- CentOS 6.5 64bit: OK (tested by alexeft on forums and anton000, v2)
- CentOS 6.5 32bit: OK (tested by anton000, v2)