i#1551 port to ARM: implement find_syscall_num and instr_is_mov_constant

Implements find_syscall_num() for ARM by sharing the x86 code via
mangle_shared.c.  Adds a new DR_REG_SYSNUM define.

Implements instr_is_mov_constant() for ARM, which is needed for
find_syscall_num().  It considers OP_mov, OP_movs, OP_movw, OP_mvn,
OP_mvns, or OP_eor where the sources equal the destination and there is no
shift (the last mainly for symmetry with x86).

Review-URL: https://codereview.appspot.com/186460043