[LLDB] Make build.py use uname to set platform

[omjavaid]

Summary

This patch makes build helper script build.py to use platform.uname for
machine/architecture detection. Visual studio environment when set using
various batch files like vcvars*.bat set PLATFORM environment variable
however VsDevCmd.bat does not set PLATFORM variable.

[DavidSpickett]

LGTM

lldb/test/Shell/helper/build.py

 #!/usr/bin/env python
 
 import argparse
 import os
+import platform
 import shutil
 import signal
 import subprocess
 import sys
 
 if sys.platform == 'win32':
     # This module was renamed in Python 3.  Make sure to import it using a
     # consistent name regardless of python version.
def build_commands(self):
             commands.append(self._get_link_command())
         return commands
 
 
 class MsvcBuilder(Builder):
     def __init__(self, toolchain_type, args):
         Builder.__init__(self, toolchain_type, args, '.obj')
 
-        if os.getenv('PLATFORM') == 'arm64':
+        if platform.uname().machine.lower() == 'arm64':
             self.msvc_arch_str = 'arm' if self.arch == '32' else 'arm64'
         else:
             self.msvc_arch_str = 'x86' if self.arch == '32' else 'x64'
 
         if toolchain_type == 'msvc':
             # Make sure we're using the appropriate toolchain for the desired
             # target type.
             compiler_parent_dir = os.path.dirname(self.compiler)