Promote powerpc64-unknown-linux-musl to tier 2 with host tools

[Gelbpunkt]

MCP: rust-lang/compiler-team#946

Tested by compiling on an x86_64 host via DEPLOY=1 ./src/ci/docker/run.sh dist-powerpc64-linux-musl and running on Alpine Linux ppc64:

root@algol /t/rust-nightly-powerpc64-unknown-linux-musl# bash install.sh
install: creating uninstall script at /usr/local/lib/rustlib/uninstall.sh
install: installing component 'rustc'
install: installing component 'rust-std-powerpc64-unknown-linux-musl'
install: installing component 'cargo'
install: installing component 'rustfmt-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-powerpc64-unknown-linux-musl'
install: installing component 'llvm-bitcode-linker-preview'

    rust installed.

root@algol /t/rust-nightly-powerpc64-unknown-linux-musl# echo 'fn main() { println!("hello world"); }' > test.rs
root@algol /t/rust-nightly-powerpc64-unknown-linux-musl# which rustc
/usr/local/bin/rustc
root@algol /t/rust-nightly-powerpc64-unknown-linux-musl# rustc test.rs
root@algol /t/rust-nightly-powerpc64-unknown-linux-musl# ./test
hello world
root@algol /t/rust-nightly-powerpc64-unknown-linux-musl# file test
test: ELF 64-bit MSB pie executable, 64-bit PowerPC or cisco 7500, OpenPOWER ELF V2 ABI, version 1 (SYSV), dynamically linked, interpreter /lib/ld-musl-powerpc64.so.1, BuildID[sha1]=90a0fa543b3d42588ad32c5b858e56ac9c56faed, with debug_info, not stripped

I renamed the glibc job and created a new one for musl since the same is done for the little-endian targets.

Implements #149938

try-job: dist-powerpc64-linux-gnu
try-job: dist-powerpc64-linux-musl

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

This PR changes how LLVM is built. Consider updating src/bootstrap/download-ci-llvm-stamp.

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

[rustbot]

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[Mark-Simulacrum]

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 328946a (328946aa062b20e80b1b51d0cfbd1bddb5e18657, parent: 8188f6c8084bade919921457e011fa39c6bf09a4)