Fix build of library archive with Arm Compiler 6

The 'archive' method of the toolchain class 'ARM' expects that self.ar
is a string, but the constructor of the class 'ARMC6' initializes it
with a list. This patch fixes the issue.

Author: miyuki

tools/toolchains/arm.py

@@ -454,7 +454,7 @@ def __init__(self, target, *args, **kwargs):
                      self.flags['common'] + self.flags['cxx'])
         self.asm = [join(TOOLCHAIN_PATHS["ARMC6"], "armasm")] + self.flags['asm']
         self.ld = [join(TOOLCHAIN_PATHS["ARMC6"], "armlink")] + self.flags['ld']
-        self.ar = [join(TOOLCHAIN_PATHS["ARMC6"], "armar")]
+        self.ar = join(TOOLCHAIN_PATHS["ARMC6"], "armar")
         self.elf2bin = join(TOOLCHAIN_PATHS["ARMC6"], "fromelf")
 
     def _get_toolchain_labels(self):