Promote powerpc64le-unknown-linux-musl to tier 2 with host tools
MCP: https://github.com/rust-lang/compiler-team/issues/803
I'm using crosstool-ng for building a toolchain because GCC 9 from `musl-toolchain.sh` has float ABI issues (?) and can't compile LLVM, and writing a crosstool-ng config for a target feels less hacky than yet another target specific shell script. I also defined a kernel version, since there wasn't one specified before. If a lower version is desired, just let me know. I also tried to match the rust configure args with the loongarch64 musl tier 2 target.
The resulting compiler works fine, built with `DEPLOY=1 ./src/ci/docker/run.sh dist-powerpc64le-linux` and tested on Alpine Linux in a VM and on a bare metal POWER8 machine:
```
qemu-ppc64le:/tmp/rust-nightly-powerpc64le-unknown-linux-musl$ ash install.sh
install: creating uninstall script at /usr/local/lib/rustlib/uninstall.sh
install: installing component 'rustc'
install: installing component 'rust-std-powerpc64le-unknown-linux-musl'
install: installing component 'cargo'
install: installing component 'rustfmt-preview'
install: installing component 'rls-preview'
install: installing component 'rust-analyzer-preview'
install: installing component 'llvm-tools-preview'
install: installing component 'clippy-preview'
install: installing component 'miri-preview'
install: installing component 'rust-analysis-powerpc64le-unknown-linux-musl'
install: installing component 'llvm-bitcode-linker-preview'
install: WARNING: failed to run ldconfig. this may happen when not installing as root. run with --verbose to see the error
rust installed.
qemu-ppc64le:~$ echo 'fn main() { println!("hello world"); }' > test.rs
qemu-ppc64le:~$ rustc test.rs
qemu-ppc64le:~$ ./test
hello world
qemu-ppc64le:~$ file test
test: ELF 64-bit LSB executable, 64-bit PowerPC or cisco 7500, OpenPOWER ELF V2 ABI, version 1 (SYSV), statically linked, BuildID[sha1]=596ee6abf9add487ebc54fb71c2076fb6faea013, with debug_info, not stripped
```
try-job: dist-powerpc64le-linux
ca5dfa74f1