Temporarily disable building the armv7 variants of profile_ios on
darwin.

After talking with Jim Grosbach pretty extensively, he was OK with me
punting on this to the Apple folks.

We agreed that the correct fix is either to extend the fake SDK headers
in compiler-rt to support the rest of libc needed by GCDAProfiling.c or
to teach the make build to test for the existence of a suitable SDK on
the system prior to building it. Both of these require someone with
access to the SDK which I don't have, and the latter (my preferred
solution) requires understanding the strange compiler-rt make build
system. Punting to Dan Dunbar or one of the others who support this
stuff on ios to write the appropriate fix, and we can let the build bots
proceed in the mean time.