rust: Ensure unwind tables are generated

Currently, we are depending on a platform default in rustc's
android_base module to keep unwind tables generated. That default was
configured specifically to change the behavior for Firefox profiling
purposes prior to the existence of the override flag. As a result,
there's no structural reason the default will remain as is.

Bug: 193702360
Test: m
Change-Id: Ic5b57d5a31f4e67a66351a3f0efc530c95aa3836