gdb: PowerPC fix gdb.base/break-interp.exp

gdb: PowerPC fix gdb.base/break-interp.exp

This patch fixes eight test failures on PowerPC for the test
gdb.base/break-interp.exp. The patch adds a funtion and registers it to
setup the displaced stepping for ppc-linux platform.  The patch moves the
struct ppc_inferior_data to the ppc-tdep.h include file to make it visible
to the ppc-linux-tdep.c and rs6000-tdep.c files.  Additionally the function
get_ppc_per_inferior is made external in ppc-tdep.h to make it visible in
both files.

Tested on Power 10 ppc64le-linux with no regressions.