Build 64-bit binaries by default when compiling for iOS
Build 64-bit binaries by default when compiling for iOS

All the bots force the value of target_subarch so the default value is only
used by developpers. Since the vast majority of the devices used on a daily
basis by developpers are 64-bit devices swap the default to 64-bit.

BUG=None

Review URL: https://codereview.chromium.org/1021513002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#323039}

Pluto-tv/chromium-crosswalk,ltilve/chromium,TheTypoMaster/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,Fireblend/chromium-crosswalk,Just-D/chromium-1,PeterWangIntel/chromium-crosswalk,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,fujunwei/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,fujunwei/chromium-crosswalk,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,Fireblend/chromium-crosswalk,fujunwei/chromium-crosswalk,ltilve/chromium,hgl888/chromium-crosswalk,fujunwei/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,ltilve/chromium,Fireblend/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,Fireblend/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,fujunwei/chromium-crosswalk,chuan9/chromium-crosswalk,fujunwei/chromium-crosswalk,axinging/chromium-crosswalk,Fireblend/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,ltilve/chromium,chuan9/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,Fireblend/chromium-crosswalk,hgl888/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Chilledheart/chromium,Fireblend/chromium-crosswalk,fujunwei/chromium-crosswalk,hgl888/chromium-crosswalk,ltilve/chromium,Just-D/chromium-1,axinging/chromium-crosswalk,ltilve/chromium,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,ltilve/chromium,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,fujunwei/chromium-crosswalk,ltilve/chromium,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,axinging/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,Chilledheart/chromium,PeterWangIntel/chromium-crosswalk,fujunwei/chromium-crosswalk,Just-D/chromium-1,PeterWangIntel/chromium-crosswalk,axinging/chromium-crosswalk,ltilve/chromium,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Fireblend/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Fireblend/chromium-crosswalk