Rollup merge of #144092 - Gelbpunkt:musl-stage0, r=Kobzol

bootstrap: Detect musl hosts

Currently, all non-Android Linux hosts are assumed to be using glibc. This obviously isn't very portable and will currently result in downloading a stage0 toolchain for glibc even on musl hosts.

There are multiple ways to detect musl somewhat reliably, but the easiest option is to check for the python target, which is exposed in `sys.implementation._multiarch` and has values like "x86_64-linux-gnu" or "powerpc64le-linux-musl".

src/bootstrap/bootstrap.py

@@ -8,6 +8,7 @@ import re
 import shutil
 import subprocess
 import sys
+import sysconfig
 import tarfile
 import tempfile
 
@@ -333,7 +334,11 @@ def default_build_triple(verbose):
         if ostype == "Android":
             kernel = "linux-android"
         else:
-            kernel = "unknown-linux-gnu"
+            python_soabi = sysconfig.get_config_var("SOABI")
+            if python_soabi is not None and "musl" in python_soabi:
+                kernel = "unknown-linux-musl"
+            else:
+                kernel = "unknown-linux-gnu"
     elif kernel == "SunOS":
         kernel = "pc-solaris"
         # On Solaris, uname -m will return a machine classification instead