[AArch64][SVE] Asm: Add SVE System registers

This patch adds system registers for controlling aspects of SVE:
- ZCR_EL1  (r/w)   visible at EL1 and EL0.
- ZCR_EL2  (r/w)   visible at EL2 and Non-secure EL1 and EL0.
- ZCR_EL3  (r/w)   visible at all exception levels.

and a system register identifying SVE:
- ID_AA64ZFR0_EL1  (r)  SVE Feature identifier.

Reviewers: SjoerdMeijer, samparker, pbarrio, fhahn, javed.absar

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D50885

llvm-svn: 340158