Make build work on OS X with older cmake versions.

`uname -p` is still i386 on OS X for some reason, which causes cmake 2.8 to set
CMAKE_SYSTEM_PROCESSOR to i386, making the build think it's doing a 32-bit
build.  However, since the system is almost completely 64-bit these days, clang
defaults to producing 64-bit object files unless told otherwise. As a result,
the produced .o files are all 64-bit except for the .o files from assembly, and
then linking fails.

Fix this by forcing ARCH to 64-bit on OS X. This matches the default behavior
of cmake 3.0, where CMAKE_SYSTEM_PROCESSOR is x86_64.

Change-Id: I7a2abc4cef84dfbaf205852a9d7b647e83dd249f
Reviewed-on: https://boringssl-review.googlesource.com/2330
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-by: Piotr Sikora <piotr@cloudflare.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index bdfaee4..d4c7a6e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -36,6 +36,13 @@
 	message(FATAL_ERROR "Unknown processor:" ${CMAKE_SYSTEM_PROCESSOR})
 endif()
 
+if (${ARCH} STREQUAL "x86" AND APPLE)
+	# With CMake 2.8.x, ${CMAKE_SYSTEM_PROCESSOR} evalutes to i386 on OS X,
+	# but clang defaults to 64-bit builds on OS X unless otherwise told.
+	# Set ARCH to x86_64 so clang and CMake agree. This is fixed in CMake 3.
+	set(ARCH "x86_64")
+endif()
+
 add_subdirectory(crypto)
 add_subdirectory(ssl)
 add_subdirectory(ssl/test)