[AArch64] Vulcan is now ThunderXT99

Broadcom Vulcan is now Cavium ThunderX2T99.

LLVM Bugzilla: http://bugs.llvm.org/show_bug.cgi?id=32113
Corresponding LLVM change: https://reviews.llvm.org/rL297190

Changes to clang to support the change.

Patch by Joel Jones

llvm-svn: 297227