gdb: move AIX_TEXT_SEGMENT_BASE to rs6000-aix-tdep.c, remove rs6000-tdep.h

gdb: move AIX_TEXT_SEGMENT_BASE to rs6000-aix-tdep.c, remove rs6000-tdep.h

The contents of rs6000-tdep.h (AIX_TEXT_SEGMENT_BASE) is AIX-specific,
so I thought that this file should be named rs6000-aix-tdep.h.  But
there's already a rs6000-aix-tdep.h, so then I though
AIX_TEXT_SEGMENT_BASE should simply be moved there, and rs6000-tdep.h
deleted.  But then I realized that AIX_TEXT_SEGMENT_BASE is only used in
rs6000-aix-tdep.c, so move it to the beginning of that file.

Change-Id: Ia212c6fae202f31aedb46575821cd642beeda7a3