A REX prefix byte must immediately precede the opcode byte or the
opcode escape byte. Emit segment override and address size prefix
bytes before a REX prefix byte.

This prevents symbol collisions when the Keystone library is linked with the LLVM libraries.

Renaming was done by the following repository-wide substitutions:
* `namespace llvm` -> `namespace llvm_ks`
* `llvm::` -> `llvm_ks::`

* add ARM Thumb branch address testcases

* fix ARM Thumb B.W addresses (issue #298)

b.cond/cbz/cbnz/tbz/tbnz/adrp

for issue #284

ARM64(llvm): Disable range checking on instructions with a PC-relative offset (b/bl/cbz/cbnz/tbz/tbnz/ldr/ldrsw/prfm)

added support to get current fragment size in the streamer interface. This fixes the address update issue in case of directive emitting values in between instructions and breaking the follow up relative jumps

use KS_ERR_NOMEM instead of -1.

This reverts commit f18b6e19.