This switches the Espressif fork from LLVM 19 to LLVM 20, so we can use
the improvements made between those LLVM versions. It also better aligns
with the system-LLVM build method, which currently also defaults to LLVM
20.
With a few small modifications, all the problems with `-gc=precise` in
WebAssembly seem to have been fixed.
I didn't do any performance measurements, but this is supposed to
improve GC performance.
Ayke van Laethem
Randy Reddig
Damian Gryski