Minor cleanup of numerical stability, constants, compound functions in OpenCL kernels

[andrjohns]

Summary

Small PR tidying the OpenCL kernels to use built-in constants and more numerically stable expressions

Tests

N/A - Existing tests should pass

Side Effects

N/A

Release notes

Minor cleanup of numerical stability, constants, compound functions in OpenCL kernels

Checklist

• Math issue #(issue number)

• Copyright holder: Andrew Johnson

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[rok-cesnovar]

Thanks!

[andrjohns]

@rok-cesnovar Would you mind re-approving when you get a minute?

[SteveBronder]

lgtm! Though we should wait to merge till after the release