Provide pid_to_exec_file on Solaris (PR tdep/17903)

Provide pid_to_exec_file on Solaris (PR tdep/17903)

While looking through gdb.log, I found that two tests FAIL like this:

warning: No executable has been specified and target does not support
determining executable automatically.  Try using the "file" command.
0x00400dc4 in ?? ()
(gdb) FAIL: gdb.base/attach.exp: attach2, with no file

The other is gdb.base/quit-live.exp.  I've implemented the following
patch that fixes both failures, only then detecting that I'd previously
reported the issue as PR tdep/17903.

Tested on amd64-pc-solaris2.10 and amd64-pc-solaris2.11.

	PR tdep/17903
	* procfs.c (procfs_target): Declare pid_to_exec_file.
	(procfs_target::pid_to_exec_file): New.