test-genzeros: allow more than 2G zeros in Windows

test-genzeros: allow more than 2G zeros in Windows

d5cfd142ec (tests: teach the test-tool to generate NUL bytes and
use it, 2019-02-14), add a way to generate zeroes in a portable
way without using /dev/zero (needed by HP NonStop), but uses a
long variable that is limited to 2^31 in Windows.

Use instead a (POSIX/C99) intmax_t that is at least 64bit wide
in 64-bit Windows to use in a future test.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>