Add messages to require statements in Math (backport #3385) (#3386)

* Add messages to require statements in Math (#3385)

* log2 functions
* unsignedBitLength

(cherry picked from commit 88fecfc1)

# Conflicts:
#	src/main/scala/chisel3/util/Math.scala

* Resolve backport conflicts

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Co-authored-by: Jack Koenig <koenig@sifive.com>