doc: document linux-x64-musl build toolchain

Add the build platform and compiler for official musl binaries.

Signed-off-by: Ji Hoon Kang <ivory.ma9ic@gmail.com>
Assisted-by: Codex
PR-URL: #66523
Fixes: #66506
Reviewed-By: Richard Lau <richard.lau@ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: theSnackOverflow

BUILDING.md

@@ -179,6 +179,7 @@ Binaries at <https://nodejs.org/download/release/> are produced on:
 | linux-ppc64le           | RHEL 9 with Clang 20.1 and gcc-toolset-14-libatomic-devel[^6] |
 | linux-s390x             | RHEL 9 with Clang 20.1 and gcc-toolset-14-libatomic-devel[^6] |
 | linux-x64               | RHEL 9 with Clang 20.1 and gcc-toolset-14-libatomic-devel[^6] |
+| linux-x64-musl          | Alpine 3.23 with Clang 21                                     |
 | win-arm64               | Windows Server 2022 (x64) with Visual Studio 2022             |
 | win-x64                 | Windows Server 2022 (x64) with Visual Studio 2022             |