RFR: 8226325: Support building of filtered spec bundles

[Dan Smith]

Fixed.

I also added a dependency for docs-specs-zip in Main.gmk, which I noticed is necessary to get gensrc to happen (these dependencies are a bit messy, but that's the status quo).

http://cr.openjdk.java.net/~dlsmith/jdk/8226325/webrev.01

> On Jun 19, 2019, at 12:21 PM, Erik Joelsson <erik.joelsson at oracle.com> wrote:
> 
> Hello Dan,
> 
> Please at least add a comment above SPEC_FILTER explaining how it can be used and the format it accepts.
> 
> Docs.gmk:492: I would recommend breaking after the =. We generally do this for (at least) multi-line macro definitions to make them stand out more. See make/common/Utils.gmk for examples.
> 
> 691-693: Please indent properly (4 spaces).
> 
> Nice that you found a way to get rid of the SPECS_BOTTOM_FILE_$n files, those were an annoying construct. Looks good except for the style nits!
> 
> /Erik
> 
> On 2019-06-18 16:05, Dan Smith wrote:
>> See this webrev for some enhancements to the documentation-building capabilities of the build system.
>> 
>> http://cr.openjdk.java.net/~dlsmith/jdk/8226325/webrev.00 <http://cr.openjdk.java.net/~dlsmith/jdk/8226325/webrev.00>
>>