test coverage for cross compiling C code and running it

[andrewrk]

Now that we have all these libcs available (See #514), we need test coverage.

• [ ] aarch64_be-linux-gnu
• [ ] aarch64_be-linux-musl
• [ ] aarch64_be-windows-gnu
• [x] aarch64-linux-gnu
• [x] aarch64-linux-musl
• [ ] aarch64-windows-gnu
• [ ] armeb-linux-gnueabi
• [ ] armeb-linux-gnueabihf
• [ ] armeb-linux-musleabi
• [ ] armeb-linux-musleabihf
• [ ] armeb-windows-gnu
• [ ] arm-linux-gnueabi
• [ ] arm-linux-gnueabihf (#3287)
• [ ] arm-linux-musleabi
• [x] arm-linux-musleabihf
• [ ] arm-windows-gnu
• [ ] i386-linux-gnu
• [ ] i386-linux-musl
• [x] i386-windows-gnu
• [ ] mips64el-linux-gnuabi64
• [ ] mips64el-linux-gnuabin32
• [ ] mips64el-linux-musl
• [ ] mips64-linux-gnuabi64
• [ ] mips64-linux-gnuabin32
• [ ] mips64-linux-musl
• [ ] mipsel-linux-gnu
• [x] mipsel-linux-musl
• [ ] mips-linux-gnu
• [ ] mips-linux-musl
• [ ] powerpc64le-linux-gnu
• [ ] powerpc64le-linux-musl
• [ ] powerpc64-linux-gnu
• [ ] powerpc64-linux-musl
• [ ] powerpc-linux-gnu
• [ ] powerpc-linux-musl
• [ ] riscv32-linux-musl
• [ ] riscv64-linux-gnu
• [ ] riscv64-linux-musl
• [ ] s390x-linux-gnu
• [ ] s390x-linux-musl
• [ ] sparc-linux-gnu
• [ ] sparcv9-linux-gnu
• [x] x86_64-linux-gnu
• [ ] x86_64-linux-gnux32
• [x] x86_64-linux-musl
• [x] x86_64-windows-gnu

The test coverage can be:

• [x] behavior tests and std lib tests, linking against libc on all these targets, and running with qemu.
• [ ] build hello_world.c against libc on all these targets, and run it with qemu.

[andrewrk]

Rather than an issue that lists all the targets, I'd rather have issues for "Tier 1 support for XYZ", and have "test coverage for libc" be one of the checkboxes.

However, I'll leave this issue open to represent adding a test for building hello_world.c against libc and running it with qemu.

[alexrp]

@andrewrk just checking since this is from 2019: Are we still interested in a C hello world that gets cross-compiled and run with QEMU for each target? Or is our current testing strategy enough?