Fix CPU architecture detection on BSDs.

CMake calls "uname" in order to detect the CPU architecture,
so $(CMAKE_SYSTEM_PROCESSOR) varies from platform to platform.

This changes adds support for "i386" and "amd64" values, which
are used by BSDs for the x86 family of CPUs.

Change-Id: I532ce787a9ac06220c92a6d8c78ad5a55d8c40bf
Signed-off-by: Piotr Sikora <piotr@cloudflare.com>
Reviewed-on: https://boringssl-review.googlesource.com/1360
Reviewed-by: Adam Langley <agl@google.com>