Provide a default target_triplet for linux on ARM32
Provide a default target_triplet for linux on ARM32

Chrome OS runs 32-bit userspace on ARM platforms. This change provides a
default target_triplet in the toolchain config to make standalone build
work for Chrome OS on ARM. Chrome OS ebuild overrides with real
target_triplet using GN args.

Bug: 147789115
Change-Id: I79e1127e8a18f84842a112e5d8791189f9ef169b
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto