First crack at adding ISA provides to packages (rhbz#235755)

First crack at adding ISA provides to packages (rhbz#235755)

- Horrible kludgery to get the isa names and bits into platform specific
  macros from installplatform script. That beast needs to die. I mean really

- In build, add provides: name(isa) = evr automatically when it makes
  sense (similarly to name = evr provides). ISA consists of ISA name and
  bitness (or wordsize). This can be used to correctly
  express multilib dependencies without resorting to (expensive!) file
  dependency kludges, eg for dlopen()'ed libraries where automatic
  dep extraction doesn't force dependency on 32bit vs 64bit version, you
  can now use:
      Requires: foo-plugin%{?_isa}
  This expands to foo-plugin(x86-32) for i?86 packages, foo-plugin(x86-64)
  to x86_64 etc, and permits spec to be shared with older distros which
  don't have ISA provides.

- The same could be expressed with "canon arch" just as well, but
  using the ISA to differentiate from %_arch and the like:
  eg i386 could be used instead of x86-32 but it's overloaded with meanings
  (the actual i386 processor vs i386 compatible cpu family etc)