Ensure JetStream2 works as a performance regression test between WPE versions on ARMv7

[eugeneia]

We want to run JetStream2 to vet changes and showcase potential performance improvements. On ARMv7 the benchmark is currently somewhat unreliable depending on the hardware und software configuration. This due to the benchmark being an atypical workload (loads of non-interactive, heavily optimized JS/Wasm code within a single "page").

We probably need to change or at least tune how we deal with memory scarcity situations to have the benchmark run reliably.

Cc @magomez

[eugeneia]

We've tested running JS2 on the sagecom board and the raspberry pi 3 and ensured it works. We landed
9ba3ac6 on the wpe-2.46 branch to make sure JSC responds to memory pressure consistently with the WPE MemoryPressureHandler. The command used to run JS2 in cog on the Rpi3 was:

WPE_RAM_SIZE=300M JSC_useUnlinkedCodeBlockJettisoning=1 cog 'https://browserbench.org/JetStream/index.html'