Make sure the effect of "pic" / "no-pic" is used with assembler compilations

Before the 'Introduce the "pic" / "no-pic" config option' commit, the
shared_cflag value for the chosen config would be part of the make
variable CFLAG, which got replicated into CFLAGS and ASFLAGS.

Since said commit, the shared_cflag value has become a make variable
of its own, SHARED_CFLAG (which is left empty in a "no-pic" build).

However, ASFLAGS was forgotten.  That's what's corrected with this
change.

Reviewed-by: Andy Polyakov <appro@openssl.org>