Add runtime support for __cpu_model (__builtin_cpu_supports)

Summary:
This aims to add support for __cpu_model and address Bug 25510. It uses
the code from lib/Support/Host.cpp for cpu detection, and creates
__cpu_model with that info.

Tested on OSX, it builts successfully, but the current version does
*not* resolve Bug 25510. The __cpu_model symbol is present in the
library but it only gets loaded with -all_load. This patch will not land
until this issue is clarified.

Built on Linux as well (though libgcc is the default). The use of "asm"
required -std=gnu99, hence the cmake change. Corrections on better
addressing this are welcome.

Note: See additional comments on D20988 (committed as r271921).

Reviewers: llvm-commits, joerg, echristo, mehdi_amini

Subscribers: mehdi_amini

Differential revision: http://reviews.llvm.org/D21033

llvm-svn: 274873