toBigInt should round identically for Double and BigDecimal (backport #3921)

[mergify]

This is an automatic backport of pull request #3921 done by Mergify.
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Original PR Body

This very minor PR fixes a bug in Num.toBigInt(x: BigDecimal, binaryPoint: Int).

The current function gives the impression to round the input BigDecimal:

val result = (x * multiplier).rounded.toBigInt

However:

BigDecimal(1.4).rounded.toBigInt // : BigInt = 1
BigDecimal(1.5).rounded.toBigInt // : BigInt = 1
BigDecimal(1.6).rounded.toBigInt // : BigInt = 1

I'm assuming this is not the intended behaviour.

The underlying issue is with the rounded function which uses the BigDecimal's MathContext. I changed this to use setScale(0) instead.

I added a testcase for rounding positive numbers. I would've added a testcase for negative ones, but as far as I know there is no RoundingMode that replicates the rounding of Doubles there (round half to positive infinity).

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Bugfix

Desired Merge Strategy

• Squash: The PR will be squashed and merged (choose this if you have no preference).

Release Notes

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
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[mergify]

Cherry-pick of b9f5703 has failed:

On branch mergify/bp/3.6.x/pr-3921
Your branch is up to date with 'origin/3.6.x'.

You are currently cherry-picking commit b9f57032b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   core/src/main/scala/chisel3/Num.scala
	both modified:   src/test/scala/chiselTests/LiteralExtractorSpec.scala

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally