loader: biosmem allocate heap just below 4GB

[tsoome]

Summary

The current biosmem code is walking bios smap entries and looking for smap
entry just below 4GB line, if there is such entry, its base and size is set
for heap base and size. Instead of entry base, we should use last HEAP_MIN
(currently 64MB) bytes just below 4GB, to make maximum space for kernel and
modules.

Illumos change: https://www.illumos.org/rb/r/661/

[emaste]

The whitespace changes make this a bit hard to read - do you have them as a separate local change that could be committed first?

[imp]

Change looks OK, but I agree with Ed, it would be better to do the white space changes separately.

[tsoome]

In D12368#256547, @emaste wrote:

The whitespace changes make this a bit hard to read - do you have them as a separate local change that could be committed first?

yes, I can do that.

[tsoome]

update after cstyle fix commit.