Add powerpc64le-unknown-linux-musl target

[runlevel5]

Summary

Adds a powerpc64le-unknown-linux-musl cross image, building the toolchain with crosstool-NG (GCC 14.2.0, musl 1.2.5), with a qemu-user runner. Closes #1776.

powerpc64le-unknown-linux-musl is a tier-2 Rust target, and the gnu counterpart already ships here; this fills the musl gap. Modeled on the existing riscv64gc-unknown-linux-musl image and #1706.

Changes

• docker/Dockerfile.powerpc64le-unknown-linux-musl — crosstool-NG build + qemu-user runner
• docker/crosstool-config/powerpc64le-unknown-linux-musl.config — crosstool-NG defconfig (powerpc64le LE, musl 1.2.5, GCC 14.2.0)
• src/docker/provided_images.rs — register the provided image
• targets.toml — declare the target (runners = "qemu-user", build-std)
• README.md — add to the supported-targets table

Validation

Built the image with docker build and exercised the toolchain:

$ powerpc64le-unknown-linux-musl-gcc --version
powerpc64le-unknown-linux-musl-gcc (crosstool-NG UNKNOWN) 14.2.0

$ powerpc64le-unknown-linux-musl-gcc -static t.c -o t && file t
t: ELF 64-bit LSB executable, 64-bit PowerPC or cisco 7500, OpenPOWER ELF V2 ABI,
   version 1 (SYSV), statically linked