bpo-37926: Fix PySys_SetArgvEx(0, NULL, 0) crash (GH-15415) (GH-15420)

bpo-37926: Fix PySys_SetArgvEx(0, NULL, 0) crash (GH-15415) (GH-15420)

empty_argv is no longer static in Python 3.8, but it is declared in
a temporary scope, whereas argv keeps a reference to it.
empty_argv memory (allocated on the stack) is reused by
make_sys_argv() code which is inlined when using gcc -O3.

Define empty_argv in PySys_SetArgvEx() body, to ensure
that it remains valid for the whole lifetime of
the PySys_SetArgvEx() call.

(cherry picked from commit c48682509dc49b43fe914fe6c502bc390345d1c2)