Warn about problematic combinations of target and host version

[taiki-e]

The "supported targets" table in the readme excludes these combinations, but it would be useful to have an explicit warning.

setup-cross-toolchain-action/.github/workflows/ci.yml

Lines 78 to 93 in 209b6d5

	exclude:
	# Segmentation fault on ubuntu 20.04
	- { target: aarch64_be-unknown-linux-gnu, os: ubuntu-20.04 }
	- { target: mips-unknown-linux-gnu, os: ubuntu-20.04 }
	- { target: mips64-unknown-linux-gnuabi64, os: ubuntu-20.04 }
	- { target: mipsisa32r6-unknown-linux-gnu, os: ubuntu-20.04 }
	- { target: mipsisa64r6-unknown-linux-gnuabi64, os: ubuntu-20.04 }
	- { target: powerpc64-unknown-linux-gnu, os: ubuntu-20.04 }
	- { target: sparc64-unknown-linux-gnu, os: ubuntu-20.04 }
	# Hang on ubuntu 18.04
	- { target: riscv64gc-unknown-linux-gnu, os: ubuntu-18.04 }
	# Toolchains not available on ubuntu 18.04
	- { target: mipsisa32r6-unknown-linux-gnu, os: ubuntu-18.04 }
	- { target: mipsisa32r6el-unknown-linux-gnu, os: ubuntu-18.04 }
	- { target: mipsisa64r6-unknown-linux-gnuabi64, os: ubuntu-18.04 }
	- { target: mipsisa64r6el-unknown-linux-gnuabi64, os: ubuntu-18.04 }

[taiki-e]

Segmentation fault on ubuntu 20.04

Maybe related to https://bugs.launchpad.net/qemu/+bug/1701798

EDIT: Hmm, the workaround discussed in that issue doesn't seem to work for our case. 9ab683a