mongodb4 on aarch64

[Greg Troxel <gdt%lexort.com@localhost>]

mongodb4 (needed for unifi) fails to build on aarch64 (as one would run
on RPI4) under NetBSD 10.

It seems there is a way for programs to find out cache line size, and a
way to tell the compiler, and then warnings (errors) if this feature is
used in a way that might end up in an ABI.

The package has:

.if ${MACHINE_ARCH} == "aarch64"
CXXFLAGS+=             --param=destructive-interference-size=64
.endif

which was needed for one person, but that flag is unknown on netbsd10
and the build fails.

My theory is that the compiler in current has more warnings, and this
setting fixes them.

Removing the CXXFLAGS+= line, it builds and packages on netbsd-10.  I do
not yet know if it works ok.  (It is fine on amd64, unsurprisingly, as
evidenced by unifi working.)

If you are using mongodb4 on aarch64, please let me know if 9, 10, or
current, and if it builds now, and once built, if it works.

I am inclined to drop this flag addition because building on 10 is more
important than current (at least for the branch), and we can add it back
based on gcc version or somehow, for systems that need it, once we
understand more about what's going on.

For 10, it is simple: gcc10 does not recognize that flag and thus the
build fails.  One doesn't need to understand anything about the grand
scheme of cache line size to follow that part.