nextpnr: add ARCH 'ecp5' (prjtrellis)

[umarcor]

As commented in https://github.com/msys2/MINGW-packages/pull/7509#issuecomment-752734673, one of the components of nextpnr-ecp5 needs more that 4GB to be build. Therefore, it cannot be done in a 32 bit memory space.

The proposed workaround is to use a script and call it in a MINGW64 shell (even on MINGW32 builds) (see https://github.com/lazka/MSYS2-packages/blob/fa0ba92d8608a0393574eb7febd26025a2a7e45b/pacman/makepkg-mingw#L111-L113). However, doing so requires x86_64-cmake and x86_64-prjtrellis to be available on the MINGW32 build. That's because MINGW32 and MINGW64 jobs are independent in CI, so the MINGW32 build cannot reuse an artifact from the MINGW64 job.

[Biswa96]

Why a separate shell script to build?

[umarcor]

@Biswa96:

one of the components of nextpnr-ecp5 needs more that 4GB to be build. Therefore, it cannot be done in a 32 bit memory space.

[umarcor]

This PR is now based on #8231.

[lazka]

Is there anything that can be done here? Or should I close it?