use ar from esp toolchain instead of host

[devsaurus]

Fixes #2444.

• This PR is for the dev branch rather than for master.
• This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• I have thoroughly tested my contribution.

The steps for preparing the binary libraries use ar from the host's toolchain. This appears to fail sometimes as reported in the related issue.
Since we're operating on archives for the target esp system, I'd say it's safer to use ar from the target xtensa toochain.

[TerryE]

Thanks, Arnim 😃