RFR: 8268174: Move x86-specific stub declarations into stubRoutines_x86.hpp

[Vladimir Ivanov]

On Thu, 3 Jun 2021 12:20:34 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> A cleanup which makes x86-specific stubs platform-specific.
> 
> Testing:
> - [x] hs-tier1 - hs-tier2
> - [x] x86_32 hotspot build

Thanks for the review, Vladimir.

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PR: https://git.openjdk.java.net/jdk/pull/4335