introduce gdb_remote_download and finish parallel fixes in gdb.dwarf2

introduce gdb_remote_download and finish parallel fixes in gdb.dwarf2

This finishes making gdb.dwarf2 parallel-safe.

To do this, this patch introduces a new gdb_remote_download proc, that
works somewhat differently in the one specific case where it matters:
for a copy to "host", if no destination was given, and the host is not
actually remote, then standard_output_file is used.  In parallel mode
this guarantees that the resulting file will end up in a parallel-safe
location.

Tested on x86-64 Fedora 18.

	* gdb.dwarf2/dw2-basic.exp: Use gdb_remote_download.
	* gdb.dwarf2/dw2-compressed.exp: Use gdb_remote_download.
	* gdb.dwarf2/dw2-intercu.exp: Use gdb_remote_download.
	* gdb.dwarf2/dw2-intermix.exp: Use gdb_remote_download.
	* gdb.dwarf2/dw2-producer.exp: Use gdb_remote_download.
	* gdb.dwarf2/mac-fileno.exp: Use gdb_remote_download.
	* lib/gdb.exp (gdb_remote_download): New proc.