[AArch64] Define ETE and TRBE system registers

Embedded Trace Extension and Trace Buffer Extension are optional
future architecture extensions.
(cf. https://developer.arm.com/architectures/cpu-architecture/a-profile/exploration-tools)

Their system registers are documented here:
https://developer.arm.com/docs/ddi0601/a

ETE shares register names with ETM. One exception is the ETE
TRCEXTINSELR0 register, which has the same encoding as the ETM
TRCEXTINSELR register (but different semantics). This patch treats
them as aliases: the assembler will accept both names, emitting
identical encoding, and the disassembler will keep disassembling
to TRCEXRINSELR.

Differential Revision: https://reviews.llvm.org/D63707

llvm-svn: 367093