lang/pypy(3): update to 5.10, add flavors to ancillary ports

• Consistently use DISTVERSION instead of PORTVERSION (the former is set, not the latter)
• Only depend on libunwind on x86 arch with FreeBSD >= 11 (fixes ignored build on arm/ppc)
• Fix python dependency with depreciation of python meta ports
• Remove PYPY_BITS from pkg-plist target, no longer used
• Remove setting of PORTVERSION, DISTVERSION is set in bsd.pypy.mk
• bsd.pypy.mk:
  • Define DISTVERSION once, to ensure consistency
  • PYPY_DIR: Handle versions with multiple digits
  • PYPY_CFFI_VER now differs in value between pypy and pypy3
• bsd.pypy.cffi.mk:
  • Add FLAVORS for pypy and pypy3 [1]
  • Consistently use PYTHON_IMPL instead of the hard coded "pypy"
  • PLIST_FILES: use direct substitution instead of PLIST_SUB

Approved by: portmgr (mat) [1]
Differential Revision: https://reviews.freebsd.org/D13675