Switch the release script to build with CMake by default (PR21561)

Switch the release script to build with CMake by default (PR21561)

It retains the possibility to use the autoconf build with a
command-line option ('-use-autoconf'), and uses that by default on Darwin since
compiler-rt requires it on that platform.

This commit also removes the "Release-64" flavour and related logic. The script
would previously do two builds unless the '-no-64bit' flag was passed, but on
my machine and from those I asked this always ended up producing two 64-bit builds,
causing much confusion.

It also removes the -build-triple option, which caused the --build= flag to
get passed to ./configure. This was presumably intended for cross-compiling,
but none of the release testers use it. If someone does want to pass it,
they can use '-configure-flags --build=foo' instead.

Differential Revision: http://reviews.llvm.org/D10715

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242331 91177308-0d34-0410-b5e6-96231b3b80d8