fdo#87119: Don't use of the broken "Software" group interpreter

FormulaGroupInterpreterSoftware is known to be broken, says moggi. We should
not use it as a fallback to OpenCL.

Not sure whether it makes sense, but let's keep it in the code for now.  Make
using it conditional on setting the environment variable
SC_ALLOW_BROKEN_SOFTWARE_INTERPRETER (to any value). Only a developer who
wants to work on it should set that.

sc::FormulaGroupInterpreter::getStatic() can now return NULL, adapt callers
accordingly.

Change-Id: I20f6e2658f556e2d4a0687f3730dccdbedb88603
Reviewed-on: https://gerrit.libreoffice.org/13397
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>