gdb.base/nested-addr.{c,exp} C++ify

gdb.base/nested-addr.{c,exp} C++ify

Adjust gdb.base/nested-addr.exp to let the testcase build and run as a
C++ program.  "print /d" is used so we get "= 1" instead of "= true"
in C++ mode.

gdb/testsuite/ChangeLog:

	* gdb.base/nested-addr.c (main): Add cast.
	* gdb.base/nested-addr.exp: Use "print /d".