Add MinGW-specific execv() override.

Add MinGW-specific execv() override.

As of 2dbc887e, shell.c employs execv(), so provide a MinGW-specific
mingw_execv() override, complementing existing mingw_execvp() and
cousins.

As a bonus, this also resolves a compilation warning due to an
execv() prototype mismatch between Linux and MinGW. Linux expects
the second argument to be (char *const *), whereas MinGW expects
(const char *const *).

Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>