readMemargFlags took its reader as a `{ u32(): number }` record, and the
validator (Reader) and translator (WasmCursor) pass class instances. The
self-hosted compiler refuses a class instance in a record-typed parameter
(SC2002), so the self-hosted LLVM emitter, the native frontend programs
and the bootstrap seed builds failed. readMemargFlags now takes the
reader's u32 function, and each caller passes `() => r.u32()`.
Chris Tate
08a3a7cf06