i#1551 port to ARM: direct exit stubs on ARM

Defines ARM-specific exit stub related defines.
Refactors the exit stub comments.

Implements insert_exit_stub_other_flags() for direct fine-grained stubs on
ARM, using raw emitting.

Adds TLS fields and fills them in for access to the fcache_return address
in a single instruction from exit stubs and gencode.

Adds ARM_INSTR_SIZE and uses it in the decoder and instr routines.
Adds ARM_CUR_PC_OFFS and uses it in the encoder.

Disables the options -separate_private_stubs, -separate_shared_stubs,
-free_private_stubs, and -pad_jmps on ARM.

Moves patch_branch() to be arch-specific and adds an initial implementation
for ARM.

Review-URL: https://codereview.appspot.com/192780043