gn: Make target_cpu=x86 v8_target_arch=arm builds generate build files.

build/config/arm.gni

Index: build/config/arm.gni
diff --git a/build/config/arm.gni b/build/config/arm.gni
index d16c0d2a3b798168a2931286583aa333e3dea8b4..ae22c2ca97a3b0ad773e8018623cd6185d892899 100644
--- a/build/config/arm.gni
+++ b/build/config/arm.gni
@@ -90,4 +90,19 @@ if (target_cpu == "arm" || current_cpu == "arm") {
   # arm64 supports only "hard".
   arm_float_abi = "hard"
   arm_use_neon = true
+} else if (target_cpu == "x86") {
+  # We support a build configuration for fuzzing where we build for 32-bit x86,
+  # but tell v8 to generate 32-bit arm code.  In this setup, some of the
+  # arm_ variables need to be set even though target_cpu is x86.
+  # (v8_target_arch is set to "arm" in that case, but that's defined in v8,
+  # and build/ can't depend on v8, so don't check for that.)
+  # The values for these variables should be the default values from the
+  # target_cpu == "arm" block above.
+  arm_version = 7
+  arm_fpu = "neon"
+  if (current_os == "android" || target_os == "android") {
+    arm_float_abi = "softfp"
+  } else {
+    arm_float_abi = "hard"
+  }
 }

[jochen (gone - plz use gerrit), 2016/06/13 15:18:40]

why not just replace the if (current_cpu == "arm" || target_cpu == "arm") with
something like if (target_cpu_bitness==32)?

then you'd also get v8_target_arch=arm64 target_arch=x64 for free

[Nico, 2016/06/13 15:23:53]

I'm guessing that the arm check is there so that the arm_* vars are only set in
arm builds. We need them to be set slightly more often (for this hybrid build
here), but not much more often. The advantage of doing it like I did is that it
makes it clear that the hybrid build always has x86 for the v8 embedder, and
that arm_version etc can't be overridden in that mode. (No bot currently needs
to be able to do that.)

I couldn't find any bots that build with current_cpu=x64 and
v8_target_arch=arm64 – is that a supported config?