[gdb/doc] Add target triplet to man files

[gdb/doc] Add target triplet to man files

After applying hash 43662968, gdb.1 and other man pages are not added
target triplet even if we configure with --target=.

It causes conflicts on some distributions.
And uninstall rules requires $(transform) variable.

gdb/doc/ChangeLog:

	* Makefile.in (transform): New variable.
	(install-man1, install-man5): Apply $(transform) to man file names.

Tested by installing both native and cross debugger.