New IBM * XL toolset.

[grafikrobot]

Proposed changes

Replace the old vacpp and xlcpp with a new ibmcxx toolset that supports the old compilers and the new "IBM XL" and "IBM Open XL" compilers.

Types of changes

What types of changes does your code introduce?

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation content changes

Checklist

• I searched the discussions
• I searched the closed and open issues
• I read the contribution guidelines
• I added myself to the copyright attributions for significant changes
• I checked that tests pass locally with my changes
• I added tests that prove my fix is effective or that my feature works
• I added necessary documentation (if appropriate)

Further comments

This replaces #422 as a more general solution.