Choose the method used to find the roots in the GC.  This only
applies to our framework GCs.  You have a choice of two
alternatives:

- ``--gcrootfinder=shadowstack``: use a so-called "shadow
  stack", which is an explicitly maintained custom stack of
  root pointers.  This is the most portable solution.

- ``--gcrootfinder=asmgcc``: use assembler hackery to find the
  roots directly from the normal stack.  This is a bit faster,
  but platform specific.  It works so far with GCC or MSVC,
  on i386 and x86-64.

You may have to force the use of the shadowstack root finder if
you are running into troubles or if you insist on translating
PyPy with other compilers like clang.
