encoding/asn1: add dynamic length integer support for base 128 integer parsing

[mchum]

encoding/asn1 parsed base 128 integers as strictly 32-bit, causing
certificates encoded in 64-bit to fail. This addresses the 64-bit issue
and maintains support for 32-bit platforms.

Fixes #36467

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[mchum]

@googlebot I fixed it.

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Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
A maintainer will review your change and provide feedback. See
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patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11 or adds a tag like "wait-release", it means that this CL will be
reviewed as part of the next development cycle. See https://golang.org/s/release
for more details.

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