This involves very few changes, all typical to WASM ports:
* WASM doesn't currently support threads or atomics so those are
disabled.
* WASM doesn't currently support exceptions so the exception
machinery is stubbed out.
* WASM doesn't (and can't) have mmap(), so an emulation library is
used. That library currently doesn't support MAP_SHARED flags,
so MAP_PRIVATE is used instead.
There is also an update to bring ECP5 bbasm CMake rules to parity
with iCE40 ones, since although it is possible to embed chipdb into
nextpnr on WASM, a 200 MB WASM file has very few practical uses.
The README is not updated and there is no included toolchain file
because at the moment it's not possible to build nextpnr with
upstream boost and wasi-libc. Boost requires a patch (merged, will
be available in boost 1.74.0), wasi-libc requires a few unmerged
patches.
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