Fixed launching in shell on haswell enabled Macs to work more than once when you do:

% lldb -x /bin/ls
(lldb) r
(lldb) r

Prior to this fix the first time it would run /usr/bin/arch with "-arch x86_64" the first time and succeed, and fail the second when the target had updated its architecture to x86_64h. We can't specify x86_64h to /usr/bin/arch, it doesn't handle it.

Also fixed it so /usr/bin/arch is only used for Apple triples.

<rdar://problem/17951312>