Support reverse stepping in gdbserver.

[Hsiangkai]

GDB supports reverse debugging for users. However, the GDB stub should have the capability to record processor states of every instruction to make the program reversible. This pull request adds the capability to decode Thumb instructions in ARMv7M to record the processor states of every instruction during stepping. It provides limited capability to reverse debugging, i.e., only support reverse stepping after stepping forward.

The feature is optional now. Users could use -R to enable the feature.

Users should use GDB 7.11 or later to enable the feature due to GDB bug 19840 (https://sourceware.org/bugzilla/show_bug.cgi?id=19840).

Reference: https://www.gnu.org/software/gdb/news/reversible.html