gdb: make inferior::m_cwd an std::string

gdb: make inferior::m_cwd an std::string

Same idea as the previous patch, but for m_cwd.

To keep things consistent across the board, change get_inferior_cwd as
well, which is shared with GDBserver.  So update the related GDBserver
code too.

Change-Id: Ia2c047fda738d45f3d18bc999eb67ceb8400ce4e