[NFCI] Change llvm-ar '-format' to be '--format'.

Upcoming versions of the toolchain (at least when I was locally testing)
drop support for the single-dashed argument type. Fixing this ahead of
time as to hopefully avoid the toolchain-release-blocking bug in future
:).

Bug: N/A
Test: Build the platform.
Change-Id: Ia2c6dcb19556f1979f9a6ecfd3a0ec58fb73e36e

cc/builder.go

@@ -746,7 +746,7 @@ func transformObjToStaticLib(ctx android.ModuleContext,
 	arCmd := "${config.ClangBin}/llvm-ar"
 	arFlags := ""
 	if !ctx.Darwin() {
-		arFlags += " -format=gnu"
+		arFlags += " --format=gnu"
 	}
 
 	if len(wholeStaticLibs) == 0 {