[WebAssembly] Fixed AsmPrinter not emitting .functype for intrinsics

Intrinsics like `memset` were not emitted as `.functype` because
WebAssemblyAsmPrinter::emitExternalDecls explicitly skips symbols
that are isIntrinsic. Removing that check doesn't work, since the symbol
from the module refers to a 4-argument `llvm.memset.p0i8.i32` rather
than the 3-argument `memset` symbol referenced in the call.
Our `WebAssemblyMCLowerPrePass` however does collect the
`memset` symbol, so the current solution is as simple as emitting
`.functype` for those.

Fixes: https://github.com/llvm/llvm-project/issues/53712

Differential Revision: https://reviews.llvm.org/D120365