[o-murphy]

Fixes #577.

WABT 1.0.42 turned on by default the features below: in include/wabt/feature.def they are false up to 1.0.41 and true in 1.0.42, and the README of 1.0.42 marks them as supported by wasm2c. The wasm2c entries still said "Requires flag"; this sets them to "1.0.42".

Key	Before	After
exceptions	flag --enable-exceptions	1.0.42
exceptionsFinal	flag --enable-exceptions	1.0.42
extendedConst	flag --enable-extended-const	1.0.42
memory64	flag --enable-memory64	1.0.42
multiMemory	flag --enable-multi-memory	1.0.42
tailCall	flag --enable-tail-call	1.0.42

Verified with one module per feature (legacy try/catch, try_table, an extended constant expression in a global, i64-addressed load/store, load/store on a second memory, a 1 000 000-deep return_call), translated by wasm2c 1.0.42 with no flags, compiled against the wasm2c runtime and run; each returned the expected value. The modules are in #577.

Not changed:

• customPageSizes: still behind --enable-custom-page-sizes, so the current entry is correct.
• threads: the WABT 1.0.42 README does not mark it as supported by wasm2c, so no entry is added (a single-threaded atomics test did translate and run with --enable-threads, but that is not enough to call it supported).

features.json validates against features.schema.json with the CI command (ajv -c ajv-formats -s features.schema.json -d features.json --strict=true).