Use gdb_sysroot for main executable on attach

Use gdb_sysroot for main executable on attach

This commit updates exec_file_locate_attach to use exec_file_find
to compute the full pathname of the main executable in some cases.
The net effect of this is that the main executable's path will be
prefixed with gdb_sysroot in the same way that shared library paths
currently are.

gdb/ChangeLog:

	* exec.c (solist.h): New include.
	(exec_file_locate_attach): Prefix absolute executable
	paths with gdb_sysroot if set.
	* NEWS: Mention that executable paths may be prepended
	with sysroot.

gdb/doc/ChangeLog:

	* gdb.texinfo (set sysroot): Document that "set sysroot" also
	applies to executable paths if supplied to GDB as absolute.