doc: document linux-x64-musl build toolchain

공식 musl 바이너리의 빌드 플랫폼과 컴파일러 정보를 추가한다.

Signed-off-by: Ji Hoon Kang <ivory.ma9ic@gmail.com>
Assisted-by: Codex

Author: theSnackOverflow

BUILDING.md

@@ -182,6 +182,7 @@ 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             |