Reenable assembly in GMP, potentially porting snippets to Thumb

[treeswift]

• (re)try building with -marm, as this MAY solve the root cause;
• update component/gmp/thumb;
• complete Thumb transition of affected snippets;
• run tests on device;
• run benchmarks on device (can be informal or high-level; to justify reenabling assembly, expected gains must be drastic, since only marginal perf increase wouldn't outweigh complexity risks).
• write up on project wiki about Thumb limitations and workarounds.

[treeswift]

Ditto nettle