Generate an error for closure cfunctions on unsupported platforms.

[maleadt]

Work around the fatal errors on ARM and PPC (#27174 and #32154) when compiling closure cfunctions, because that blocks nightlies from getting generated. The LLVM PPC back-end does seem to support trampolines though, so we could probably fix that if somebody cares.

[vchuravy]

x-ref: #32154

Also I am fine with this, but I would like to eventually have this fixed. Do you have a reference for the PPC support?

[maleadt]

Hmm, looked like the tests timed out: https://build.julialang.org/#/builders/28/builds/7025
Trying to reproduce locally now. Is there a way to use #34212 on the bots when stuff timeouts?

@vchuravy: Do you have a reference for the PPC support?

https://github.com/llvm/llvm-project/blob/995c18fc5051850782b1c096233867b8e56e0dea/llvm/lib/Target/PowerPC/PPCISelLowering.h#L1060-L1061

[maleadt]

Maybe it's just very slow?

Currently running: 
SparseArrays/sparse (running for 73 minutes)
SparseArrays/sparsevector (running for 72 minutes)
LinearAlgebra/triangular (running for 67 minutes)
LinearAlgebra/qr (running for 61 minutes)

@staticfloat Can we bump the 2700 seconds to something higher for aarch64 and try again?

[staticfloat]

JuliaCI/julia-buildbot@a5b45cc

I've re-started your build here: https://build.julialang.org/#/builders/28/builds/7083

[maleadt]

Looks like #34155 broke something. Rebased right before that, currently running at https://build.julialang.org/#/builders/28/builds/7103