Some ARM environments don't support |getauxval| or signals and need to
configure the capabilities of the chip at compile time. This change adds
defines that allow them to do so.
Change-Id: I4e6987f69dd13444029bc7ac7ed4dbf8fb1faa76
Reviewed-on: https://boringssl-review.googlesource.com/6280
Reviewed-by: Adam Langley <agl@google.com>