doc: add arm64-musl info into platform list

[kleisauke]

See:
https://github.com/nodejs/unofficial-builds/blob/main/recipes/arm64-musl/Dockerfile

Follow-up to #61569.

Note that I considered adding riscv64 here as well, e.g.:

--- a/BUILDING.md
+++ b/BUILDING.md
@@ -116,6 +116,7 @@ platforms. This is true regardless of entries in the table below.
 | GNU/Linux        | ppc64le >=power9 | kernel >= 4.18[^1], glibc >= 2.28 | Tier 2       | e.g. Ubuntu 20.04, RHEL 8            |
 | GNU/Linux        | s390x >=z14      | kernel >= 4.18[^1], glibc >= 2.28 | Tier 2       | e.g. RHEL 8                          |
 | GNU/Linux        | loong64          | kernel >= 5.19, glibc >= 2.36     | Experimental |                                      |
+| GNU/Linux        | riscv64          | kernel >= 6.8, glibc >= 2.39      | Experimental | e.g. Ubuntu 24.04                    |
 | Windows          | x64              | >= Windows 10/Server 2016         | Tier 1       | [^2],[^3]                            |
 | Windows          | arm64            | >= Windows 10                     | Tier 2       |                                      |
 | macOS            | x64              | >= 13.5                           | Tier 1       | For notes about compilation see [^4] |

but there's already a PR open for that, see #42251.

[nodejs-github-bot]

Review requested:

• @nodejs/build
• @nodejs/tsc

[sxa]

but there's already a PR open for that, see #42251.

Yeah I was thinking about that one too when I wrote the previous comment. The unofficial builds were broken on there for a while but I've now got them mostly working again so now's the right time to look at pushing that one forward again :-)