The built-in CMake support seems to basically work, though it believes
you want to build a fat binary which doesn't work with how we build
perlasm. (We'd need to stop conditioning on CMAKE_SYSTEM_PROCESSOR at
all, wrap all the generated assembly files in ifdefs, and convince the
build to emit more than one. Probably not worth bothering for now.)

We still, of course, need to actually test the assembly on iOS before
this can be shipped anywhere.

BUG=48

Change-Id: I6ae71d98d706be03142b82f7844d1c9b02a2b832
Reviewed-on: https://boringssl-review.googlesource.com/14645
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>