Fix symbol table file name on AIX

Fix symbol table file name on AIX

When xlc -qfuncsect or gcc -ffunction-sections options is used in AIX,
each function csect is associated with each psymtab, so each psymtab
will have it's corresponding filename entries set.

If the pst filename isn't set then we will be seeing the below output
when we set a breakpoint.

(gdb) br main
Breakpoint 1 at 0x10000374: file  _start_ , line 18.

With the fix it will be.

(gdb) br main
Breakpoint 1 at 0x10000518: file test.c, line 24.

Attached patch resolve this issue and correct filename will be set.

gdb/
2016-10-21  Sangamesh Mallayya  <sangamesh.swamy@in.ibm.com>
	    Ulrich Weigand  <uweigand@de.ibm.com>

	* xcoffread.c (read_xcoff_symtab): Make name of current file as
	pst->filename instead of _start_ in AIX.

Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>