Update get_standard_cache_dir for macOS

On macOS the usual cache directory is ~/Library/Caches.  This patch
changes get_standard_cache_dir to use that instead of XDG.

gdb/ChangeLog
2018-09-17  Tom Tromey  <tom@tromey.com>

	* common/pathstuff.c (get_standard_cache_dir): Use
	~/Library/Caches on macOS.
	* common/pathstuff.h (get_standard_cache_dir): Update comment.

gdb/doc/ChangeLog
2018-09-17  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Index Files): Update for cache directory change on
	macOS.