Feature Request: absorb libpgmath into main flang target

[naromero77]

@gklimowicz @sscalpone

In order to build classic flang, https://github.com/flang-compiler/flang/wiki/Building-Flang

You have to do a make in the runtime/libpgmath directory, before the main make in the top level directory. This makes the packaging classic Flang in the DOE Spack manager a bit more complicated -- because by default you are only allowed one make invocation in a Spack package (although there are some convoluted ways around this). Is there any reason we cannot include runtime/libpgmath as part of the main Flang target in CMake?