Fix tests, linkers, and specifications for various targets.

[Alexhuszagh]

• Fixes linking to libgcc for armv5te-unknown-linux-musleabi.
• Adds C++ support for FreeBSD targets by linking to libstdc++.
• Tests dynamic library support for Android targets in CI.
• Tests C++ support for mips64el-unknown-linux-muslabi64 in CI.
• Convert mips64el-unknown-linux-muslabi64 to a hard-float toolchain to match the rust target.
• Convert mips64el-unknown-linux-muslabi64 to use the mips64r2 architecture.
• Convert mips-unknown-linux-musl and mipsel-unknown-linux-musl to use the mips32r2 architecture, identical to the rust targets.
• Document whether MIPS musl targets are hard or soft float.

[Alexhuszagh]

Blocking this until #1008 is merged, so we can use the new TOML format for the matrix.

[Alexhuszagh]

bors try

[Alexhuszagh]

We might be able to fix the partial C++ support for mips64el-unknown-linux-muslabi64 by checking the Rustc version in newer releases. If not, we can always link to libgcc like we do for mips64-unknown-linux-muslabi64. This might be the preferable solution.

[bors]

try

Build failed:

• conclusion

[Alexhuszagh]

bors try --target mips64el-unknown-linux-muslabi64

[bors]

try

Build succeeded:

• conclusion

[Alexhuszagh]

This is technically a breaking change for mips64el-unknown-linux-muslabi64, however, this changes our build to match the Rust target itself, and since we're moving to a 0.3.0 release, we're allowed to have breaking changes here:
https://github.com/cross-rs/cross/pull/1028/files#diff-8541a3a4775c5542e352ca560a66b3bb939d2dc6112554657dfd8930b87e60ff

Similarly, the 32-bit MIPS musl targets should also likely be hard-float.

[Alexhuszagh]

We should clearly document the target specifications, since we know:

• mips-unknown-linux-musl: soft-float (mips_unknown_linux_musl.rs#L7)
• mipsel-unknown-linux-musl: soft-float (mipsel_unknown_linux_musl.rs#L6)
• mips64-unknown-linux-muslabi64: hard-float (mips64_unknown_linux_muslabi64.rs#L7)
• mips64el-unknown-linux-muslabi64: hard-float (mips64el_unknown_linux_muslabi64.rs#L6)

So with this change, all our targets are now correct for soft or hard-float.

[Alexhuszagh]

We can probably get away with building the musl toolchain for a generic mips32 and mips64 architecture, however, since the Rust target will use a more specific architecture, I doubt this will have any changes in practice. This will (minorly) affect external C or C++ dependencies, but shouldn't affect any Rust code.

[Alexhuszagh]

bors try --target mipsmusl

[bors]

try

Build succeeded:

• conclusion

[Emilgardis]

ping @abcfy2

does this change work for you?
You can build the image with

cargo xtask build-docker-image mips64el-unknown-linux-muslabi64

this is a hard-float target according to rustc

[Alexhuszagh]

It should be hard-float, we're just using a soft float toolchain which is bound to cause problems at some point. A quick test is:

$ cargo init --bin hello
$ cross build --target mips64el-unknown-linux-muslabi64
$ file target/mips64el-unknown-linux-muslabi64/debug/hello
target/mips64el-unknown-linux-muslabi64/debug/hello: ELF 64-bit LSB executable, MIPS, MIPS64 rel2 version 1 (SYSV), statically linked, with debug_info, not stripped
$ readelf -A target/mips64el-unknown-linux-muslabi64/debug/hello | grep Tag_GNU_MIPS_ABI_FP
  Tag_GNU_MIPS_ABI_FP: Hard float (double precision)
$ docker run -it --rm ghcr.io/cross-rs/mips64el-unknown-linux-muslabi64:main bash
# still using the soft-float toolchain
$ echo $CROSS_MUSL_SYSROOT/
/usr/local/mips64el-linux-muslsf/

# show 32-bit is soft-float
$ cross build --target mipsel-unknown-linux-musl
$ file target/mipsel-unknown-linux-musl/debug/hello
target/mipsel-unknown-linux-musl/debug/hello: ELF 32-bit LSB pie executable, MIPS, MIPS32 rel2 version 1 (SYSV), dynamically linked, interpreter /lib/ld-musl-mipsel-sf.so.1, with debug_info, not stripped
$ readelf -A target/mipsel-unknown-linux-musl/debug/hello | grep Tag_GNU_MIPS_ABI_FP
  Tag_GNU_MIPS_ABI_FP: Soft float

So we're linking a hard-float executable with a soft-float toolchain, which isn't exactly ideal, and bound to cause problems later down the line.

[Alexhuszagh]

bors try --target mipsmusl --target aarch64-unknown-linux-musl

[Emilgardis]

looks good!

[bors]

try

Build succeeded:

• conclusion

[Alexhuszagh]

bors r=Emilgardis

[bors]

Build succeeded:

• conclusion