Disable 32-bit Darwin builds

Mac OSX 10.14 Mojave deprecates 32-bit executables, and the 10.14 SDK
does not successfully link 32-bit files.

Test: Install Xcode 10 beta, `m checkbuild host`
Change-Id: Ifad471fb88ead4bda870f49837d937fb5742f3f6
diff --git a/core/envsetup.mk b/core/envsetup.mk
index 39b9552..161080b 100644
--- a/core/envsetup.mk
+++ b/core/envsetup.mk
@@ -147,6 +147,11 @@
 endif
 endif
 
+ifeq ($(HOST_OS),darwin)
+  # Mac no longer supports 32-bit executables
+  HOST_2ND_ARCH :=
+endif
+
 BUILD_ARCH := $(HOST_ARCH)
 BUILD_2ND_ARCH := $(HOST_2ND_ARCH)