Add LLVM target and cputype matching for loongarch64

Author: zhaixiaojuan

src/bootstrap/bootstrap.py

@@ -304,6 +304,7 @@ def default_build_triple(verbose):
         'i486': 'i686',
         'i686': 'i686',
         'i786': 'i686',
+        'loongarch64': 'loongarch64',
         'm68k': 'm68k',
         'powerpc': 'powerpc',
         'powerpc64': 'powerpc64',