First step in avoiding compilation/usage of non-relevant targets. New

First step in avoiding compilation/usage of non-relevant targets. New
options have been added to the configure script that control which targets
will be used. The options are:

--enable-target-this (default=disabled)
  This will specify that the target corresponding to the build host is
  the target that will be compiled/used. You can't use this with any of
  the other options (they'll be ignored). This is what most people want.

--disable-target-x86 (default=enabled)
  This will prevent the X86 target(s) from being compiled/used.

--disable-target-sparc (default=enabled)
  This will prevent both SparcV8 and SparcV9 from being compiled/used.

--disable-target-powerpc (default=enabled)
  This will prevent the PowerPC target from being compiled/used.

--disable-target-alpha (default=enabled)
  This will prevent the Alpha target from being compiled/used.

--disable-target-ia64 (default=enabled)
  This will prevent the IA64 target from being compiled/used.

Note that without any of these options, the default behavior is to build
all targets, as is the current practice.

All these options do is set up the substititution variable TARGETS_TO_BUILD
which contains the targets that should be compiled/used. The variable is
intended to be used in the makefiles. Those changes will come later.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@21445 91177308-0d34-0410-b5e6-96231b3b80d8