Fix bootstrap.py uname error.

The x.py script fails with `ValueError: too many values to unpack (expected 3)` when uname -smp gives more than 3 words

src/bootstrap/bootstrap.py

@@ -256,7 +256,7 @@ def default_build_triple(verbose):
     if uname is None:
         return 'x86_64-pc-windows-msvc'
 
-    kernel, cputype, processor = uname.decode(default_encoding).split()
+    kernel, cputype, processor = uname.decode(default_encoding).split(maxsplit=2)
 
     # The goal here is to come up with the same triple as LLVM would,
     # at least for the subset of platforms we're willing to target.