infozip: fix $(AS) behavior

infozip's Makefile expect $(AS) to output by default to <basename>.o
but when called from Buildroot, the value for $(AS) is set
to to invoke the toolchain's $(AS) directly, and the output of it
goes to a.out.

So, define $(AS) with $(CC -c)

Reported-by: Кирилл Луценко <lucenko_kirill@mail.ru>
Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>