Bug 1271829 - Propagate MSVC -arch switch to nss. r=glandium

This updates the nss build to require SSE2 instructions on win32.

By default nss builds with -arch:IA32 for maximum compatibility.
Firefox 49 dropped support for non-SSE2 platforms, so propagate
that to the nss external build. XCFLAGS come after the nss build's
default settings, and the last -arch switch will override the
previous ones.

Bump CLOBBER because we've had trouble with file removal permission
errors on integration builds.

MozReview-Commit-ID: 2lfDq0e0T7Y

CLOBBER

@@ -22,4 +22,4 @@
 # changes to stick? As of bug 928195, this shouldn't be necessary! Please
 # don't change CLOBBER for WebIDL changes any more.
 
-Bug 1273625 - Update win32 rustc target to i686
+Bug 1271829 - Compile nss with SSE2 optimizations

config/external/nss/Makefile.in

@@ -271,6 +271,13 @@ ifdef MOZ_FOLD_LIBS_FLAGS
 XCFLAGS += $(MOZ_FOLD_LIBS_FLAGS)
 endif
 
+# Pass on the MSVC target arch from the main build system.
+# Note this is case- and switch-character sensitive, while
+# the MSVC option is not.
+ifeq (WINNT,$(OS_TARGET))
+XCFLAGS += $(filter -arch:%,$(CFLAGS))
+endif
+
 # Export accumulated XCFLAGS to modify nss defaults.
 DEFAULT_GMAKE_FLAGS += XCFLAGS='$(XCFLAGS)'