lang/python35: Fix packaging on GCC architectures

[pkubaj]

Summary

_pickle.so isn't build with base GCC by default:
/usr/local/poudriere/ports/default/lang/python35/work/Python-3.5.7/Modules/_pickle.c: In function 'PyMemoTable_Copy':
/usr/local/poudriere/ports/default/lang/python35/work/Python-3.5.7/Modules/_pickle.c:677: error: 'for' loop initial declaration used outside C99 mode
/usr/local/poudriere/ports/default/lang/python35/work/Python-3.5.7/Modules/_pickle.c: In function '_pickle_PicklerMemoProxy_copy_impl':
/usr/local/poudriere/ports/default/lang/python35/work/Python-3.5.7/Modules/_pickle.c:4207: error: 'for' loop initial declaration used outside C99 mode
/usr/local/poudriere/ports/default/lang/python35/work/Python-3.5.7/Modules/_pickle.c: In function 'Unpickler_set_memo':
/usr/local/poudriere/ports/default/lang/python35/work/Python-3.5.7/Modules/_pickle.c:6794: error: 'for' loop initial declaration used outside C99 mode
/usr/local/poudriere/ports/default/lang/python35/work/Python-3.5.7/Modules/_pickle.c:6842: error: 'for' loop initial declaration used outside C99 mode

Set USE_CSTD=c99 to fix build.

PR: 238821

[mat]

make sure the commit message contains every bit of information that was requested in the PR.