Build prebuilt libraries for alternate host target

Having a prebuilt for i686 will allow us to handle host binaries
consistently rather than using in-tree std for 32-bit host but prebuilt
for 64-bit host.

Bug: 169347277
Change-Id: I0cf7378364663592f505b7c26065e1bc5ab7cb8c

Author: maurer

build_platform.py

@@ -33,3 +33,12 @@ def triple():
     if build_os == 'darwin':
         return 'x86_64-apple-darwin'
     raise RuntimeError("Unknown OS: " + build_os)
+
+def alt_triples():
+    """Returns the multilib targets for the build environment."""
+    build_os = system()
+    if build_os == 'linux':
+        return ['i686-unknown-linux-gnu']
+    if build_os == 'darwin':
+        return []
+    raise RuntimeError("Unknown OS: " + build_os)

config_toml.py

@@ -20,7 +20,7 @@
 import build_platform
 import paths
 
-host_targets = [build_platform.triple()]
+host_targets = [build_platform.triple()] + build_platform.alt_triples()
 device_targets = ['aarch64-linux-android', 'armv7-linux-androideabi',
                   'x86_64-linux-android', 'i686-linux-android']
 all_targets = host_targets + device_targets