Avoiding OOM for armv7 ports on arm64

Greetings,

One thing that bugged me for quite a while is that armv7 tasks on arm64
hosts (common case for cross-building ports) can seemingly only allocate
2 GB of memory despite having access to a full 32 bit virtual address
space, which should give 4 GB of memory to play around with.

This causes many ports to fail due to address space exhaustion in LLVM
and rustc.

Debugging this with kib, we found that poor default and lack of kernel
code are to blame (PR #274705).  A workaround can be applied to select
roughly the same defaults as used for i386 tasks on amd64:

To /etc/sysctl.conf, add

    kern.maxssiz=67108864
    kern.maxdsiz=536870912

to restrict the stack reservation to 64M.  This also affects 64 bit
processes, but I don't think anybody needs more stack than that.
Data is restricted to 512M, which doesn't really matter for 64 bit
as we do not support sbrk on arm64.

Then reboot, or apply these limits with the sysctl utility.

Hope this helps!

Yours,
Robert Clausecker

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