Fix signedness error of max LOS size in C2 (backport #348)

[mergify]

The PlanConstraints::max_non_los_default_alloc_bytes constant is a usize and can be usize::MAX for some plans including NoGC. The C2 code erroneously considered it signed and did signed comparison against object sizes. Consequently, when allocating dynamically sized arrays, the C2-generated code always goes to the slow path. It slows down certain workloads (such as hsqldb and luindex in DaCapo 2006) by 20% in total time.

We fix this bug by always performing 64-bit unsigned comparison.

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This is an automatic backport of pull request #348 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   mmtk/Cargo.toml
	modified:   openjdk/mmtkBarrierSetC2.cpp

Unmerged paths:
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	both modified:   mmtk/Cargo.lock

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