[AArch64][ELF] Add support for PLT decoding with BTI instructions present

Arm Architecture v8.5a introduces Branch Target Identification (BTI). When
enabled all indirect branches must target a bti instruction of the
appropriate form. As PLT sequences may sometimes be the target of an
indirect branch and PLT[0] always is, a static linker may need to generate
PLT sequences that contain "bti c" as the first instruction. In effect:
bti     c
adrp    x16, page offset to .got.plt
...
Instead of:
adrp    x16, page offset to .got.plt
...
At present the PLT decoding assumes the adrp will always be the first
instruction. This patch adds support for a single "bti c" to prefix it. A
test binary has been uploaded with such a PLT sequence. A forthcoming LLD
patch will make heavy use of the PLT decoding code.

Differential Revision: https://reviews.llvm.org/D62598

llvm-svn: 362523