scalar-crypto: Initial spike support for v0.8.1 (#635)

Brief:

- This commit adds spike support for the scalar cryptography extension.
  See the riscv-crypto repository (https://github.com/riscv/riscv-crypto)
  for more information on this extension.

- It is based on the experimental patch which has so far been kept in the
  riscv-crypto repository. Now that scalar crypto is nearly at the "freeze"
  stage and entering opcode consistency review, it makes sense to start
  upstreaming our experimental version.

- In terms of compiler support - we are using an experimental patch in
  the riscv-crypto repository at the moment, others are working on an
  upstream appropriate version.

Details:

- Add support for dedicated scalar crypto instructions.

- Add very basic support for the entropy source (entropy_source.h).
  Much of the behaviour of this is implementation specific. This model
  aims to provide the bare minimum of functionality which can be used to
  quickly develop software. It uses /dev/urandom as its entropy source for
  now.

- Scalar crypto is unique in that it _borrows_ instructions from the
  Bitmanipulation extension. This is currently _not_ encoded in the patch,
  as I didn't want to damage anything in Bitmanip which is currently under
  review. However, I've added a macro in riscv/decode.h called
  "require_either_extension(A,B)", which allows instructions to be valid
  opcodes iff they are in one or both extensions.

 On branch scalar-crypto
 Changes to be committed:
	modified:   README.md
	modified:   riscv/decode.h
	modified:   riscv/encoding.h
	new file:   riscv/entropy_source.h
	new file:   riscv/insns/aes64im.h
	new file:   riscv/insns/aes64ks1i.h
	new file:   riscv/insns/aes64ks2.h
	new file:   riscv/insns/aes_common.h
	new file:   riscv/insns/aesds.h
	new file:   riscv/insns/aesdsm.h
	new file:   riscv/insns/aeses.h
	new file:   riscv/insns/aesesm.h
	new file:   riscv/insns/sha256sig0.h
	new file:   riscv/insns/sha256sig1.h
	new file:   riscv/insns/sha256sum0.h
	new file:   riscv/insns/sha256sum1.h
	new file:   riscv/insns/sha512sig0.h
	new file:   riscv/insns/sha512sig0h.h
	new file:   riscv/insns/sha512sig0l.h
	new file:   riscv/insns/sha512sig1.h
	new file:   riscv/insns/sha512sig1h.h
	new file:   riscv/insns/sha512sig1l.h
	new file:   riscv/insns/sha512sum0.h
	new file:   riscv/insns/sha512sum0r.h
	new file:   riscv/insns/sha512sum1.h
	new file:   riscv/insns/sha512sum1r.h
	new file:   riscv/insns/sm3p0.h
	new file:   riscv/insns/sm3p1.h
	new file:   riscv/insns/sm4_common.h
	new file:   riscv/insns/sm4ed.h
	new file:   riscv/insns/sm4ks.h
	modified:   riscv/processor.cc
	modified:   riscv/processor.h
	modified:   riscv/riscv.mk.in