Handle Windows drives in auto-load script paths

Handle Windows drives in auto-load script paths

Fixes this testsuite fail on Windows:
FAIL: gdb.base/auto-load.exp: print $script_loaded

Converts the debugfile path from c:/dir/file to /c/dir/file, so it can be
appended to the auto-load path.

gdb/ChangeLog:

2020-07-08  Hannes Domani  <ssbssa@yahoo.de>

	* auto-load.c (auto_load_objfile_script_1): Convert drive part
	of debugfile path on Windows.

gdb/doc/ChangeLog:

2020-07-08  Hannes Domani  <ssbssa@yahoo.de>

	* gdb.texinfo: Document Windows drive conversion of
	'set auto-load scripts-directory'.