Support 64-bit builds and restarting build process

Two changes to support 64-bit builds, based on detecting machine
hardware name as 'x86_64':
 -  change gmp build flags from "ABI=32" to
    "--enable-cxx --build=x86_64-linux"
 -  link a 'lib64' folder to 'lib' to fix a problem with libnettle
    and libhogweed not being found for 64-bit builds of 'gnutls'

Three changes to allow the build process to be restarted if a failure
occurs while building:
 - Supply a workaround for a known bug in pkg-config 0.28, which has
   been fixed upstream to remove a file created during the build
 - Supply "git pull" alternative to "git clone" where the target
   folder already exists

Change-Id: I1177f3ae55d4c1762287ed21ed68499290881f10
Signed-off-by: Roger Stringer <roger.stringer@hisense-usa.com>