Removed `<intro-instruction-set>` feature.

* template/x86_64-unknown-linux-gnu/site-config.jam: Added `generic` value
  to `<instruction-set>` feature.
* jamroot:
  * Removed the definitions of `<intro-instruction-set>` feature and
    `USE_INSTRUCTION_SET` constant.
  * Removed the usage of `<intro-instruction-set>` feature.
* compilers.jam: Removed unused rules and the usage of
  `<intro-instruction-set>` feature.
* gmp/jamfile: Removed the usage of `USE_INSTRUCTION_SET` hook.
* mpfr/jamfile: Removed the usage of `USE_INSTRUCTION_SET` hook.
* mpc/jamfile: Removed the usage of `USE_INSTRUCTION_SET` hook.
* isl/jamfile: Removed the usage of `USE_INSTRUCTION_SET` hook.
* cloog/jamfile: Removed the usage of `USE_INSTRUCTION_SET` hook.
* ppl/jamfile: Removed the usage of `USE_INSTRUCTION_SET` hook.
* icu4c/jamfile: Removed the usage of `USE_INSTRUCTION_SET` hook.
* openmpi/jamfile: Removed the usage of `USE_INSTRUCTION_SET` hook.
* boost/jamfile: Removed the usage of `<intro-instruction-set>` feature and
  `USE_INSTRUCTION_SET` hook.
* clang/jamfile: Removed the usage of `USE_INSTRUCTION_SET` hook.
* valgrind/jamfile: Removed the usage of `USE_INSTRUCTION_SET` hook.
Removed `<intro-address-model>` feature.

* jamroot:
  * Removed the definitions of `intro-address-model` feature and
    `USE_ADDRESS_MODEL` constant.
  * Removed the usage of `intro-address-model` feature.
* compilers.jam: Removed the usage of `intro-address-model` feature.
* gmp/jamfile: Removed the usage of `USE_ADDRESS_MODEL` hook.
* mpfr/jamfile: Removed the usage of `USE_ADDRESS_MODEL` hook.
* mpc/jamfile: Removed the usage of `USE_ADDRESS_MODEL` hook.
* isl/jamfile: Removed the usage of `USE_ADDRESS_MODEL` hook.
* cloog/jamfile: Removed the usage of `USE_ADDRESS_MODEL` hook.
* ppl/jamfile: Removed the usage of `USE_ADDRESS_MODEL` hook.
* icu4c/jamfile: Removed the usage of `USE_ADDRESS_MODEL` hook.
* openmpi/jamfile: Removed the usage of `intro-address-model` feature and
  `USE_ADDRESS_MODEL` hook.
* boost/jamfile: Removed the usage of `intro-address-model` feature and
  `USE_ADDRESS_MODEL` hook.
* clang/jamfile: Removed the usage of `USE_ADDRESS_MODEL` hook.
* valgrind/jamfile: Removed the usage of `USE_ADDRESS_MODEL` hook.
Merge pull request #49 from Cryolite/feature/replaceAddressModelAndInstructionSet

Replacing `<intro-address-model>` and `<intro-instruction-set>` with `<address-model>` and `<instruction-set>` respectively