Add -Wno-unused-command-line-argument to clang cflags to enable ccache.

Previously ccache is disabled when it fails calling clang's preprocessor with
unused arguments (such as '-Wa,--noexecstack') in the command line.

See http://petereisentraut.blogspot.com/2011/05/ccache-and-clang.html.
(-Qunused-arguments suppresses more than
-Wno-unused-command-line-argument does.)

Change-Id: I6cde307632c8395c053eb28063d7844d93070562
Merge "Add -Wno-unused-command-line-argument to clang cflags to enable ccache."
Reset LOCAL_2ND_ARCH_VAR_PREFIX before referencing it.

Change-Id: I9182ce1efe09609100d19c9fae2bc4fe7358dfc0
Merge "Reset LOCAL_2ND_ARCH_VAR_PREFIX before referencing it."
Set default to Clang integrated assembler.

BUG: 17820427
Change-Id: I10a6ddd94aad083665164d7939990e148b3b6efb
Merge "Set default to Clang integrated assembler."
Merge pull request #1 from android/master

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