Add support for CMAKE_OSX_ARCHITECTURES (pytorch#51)

* Add support for CMAKE_OSX_ARCHITECTURES

Test Plan:
On x86_64 Mac:
 cmake -DCMAKE_OSX_ARCHITECTURES=arm64 -DCPUINFO_BUILD_BENCHMARKS=OFF .. -G Ninja
 ninja
On M1 Mac:
  cmake -DCMAKE_OSX_ARCHITECTURES=x86_64 -DCPUINFO_BUILD_BENCHMARKS=OFF .. -G Ninja
  ninja
Then copy and execute build artifacts on a different platform

* Incorporate review feedback

CMakeLists.txt

@@ -46,6 +46,12 @@ MACRO(CPUINFO_TARGET_RUNTIME_LIBRARY target)
   ENDIF()
 ENDMACRO()
 
+# -- [ Determine target processor
+SET(CPUINFO_TARGET_PROCESSOR "${CMAKE_SYSTEM_PROCESSOR}")
+IF(CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND CMAKE_OSX_ARCHITECTURES MATCHES "^(x86_64|arm64)$")
+  SET(CPUINFO_TARGET_PROCESSOR "${CMAKE_OSX_ARCHITECTURES}")
+ENDIF()
+
 # ---[ Build flags
 SET(CPUINFO_SUPPORTED_PLATFORM TRUE)
 IF(NOT CMAKE_SYSTEM_PROCESSOR)
@@ -55,9 +61,9 @@ IF(NOT CMAKE_SYSTEM_PROCESSOR)
       "cpuinfo will compile, but cpuinfo_initialize() will always fail.")
     SET(CPUINFO_SUPPORTED_PLATFORM FALSE)
   ENDIF()
-ELSEIF(NOT CMAKE_SYSTEM_PROCESSOR MATCHES "^(i[3-6]86|AMD64|x86(_64)?|armv[5-8].*|aarch64|arm64)$")
+ELSEIF(NOT CPUINFO_TARGET_PROCESSOR MATCHES "^(i[3-6]86|AMD64|x86(_64)?|armv[5-8].*|aarch64|arm64)$")
   MESSAGE(WARNING
-    "Target processor architecture \"${CMAKE_SYSTEM_PROCESSOR}\" is not supported in cpuinfo. "
+    "Target processor architecture \"${CPUINFO_TARGET_PROCESSOR}\" is not supported in cpuinfo. "
     "cpuinfo will compile, but cpuinfo_initialize() will always fail.")
   SET(CPUINFO_SUPPORTED_PLATFORM FALSE)
 ENDIF()
@@ -113,7 +119,7 @@ SET(CPUINFO_SRCS
   src/cache.c)
 
 IF(CPUINFO_SUPPORTED_PLATFORM)
-  IF(NOT CMAKE_SYSTEM_NAME STREQUAL "Emscripten" AND (CMAKE_SYSTEM_PROCESSOR MATCHES "^(i[3-6]86|AMD64|x86(_64)?)$" OR IOS_ARCH MATCHES "^(i386|x86_64)$"))
+  IF(NOT CMAKE_SYSTEM_NAME STREQUAL "Emscripten" AND (CPUINFO_TARGET_PROCESSOR MATCHES "^(i[3-6]86|AMD64|x86(_64)?)$" OR IOS_ARCH MATCHES "^(i386|x86_64)$"))
     LIST(APPEND CPUINFO_SRCS
       src/x86/init.c
       src/x86/info.c
@@ -134,7 +140,7 @@ IF(CPUINFO_SUPPORTED_PLATFORM)
     ELSEIF(CMAKE_SYSTEM_NAME MATCHES "^(Windows|CYGWIN|MSYS)$")
       LIST(APPEND CPUINFO_SRCS src/x86/windows/init.c)
     ENDIF()
-  ELSEIF(CMAKE_SYSTEM_PROCESSOR MATCHES "^(armv[5-8].*|aarch64|arm64)$" OR IOS_ARCH MATCHES "^(armv7.*|arm64.*)$")
+  ELSEIF(CPUINFO_TARGET_PROCESSOR MATCHES "^(armv[5-8].*|aarch64|arm64)$" OR IOS_ARCH MATCHES "^(armv7.*|arm64.*)$")
     LIST(APPEND CPUINFO_SRCS
       src/arm/uarch.c
       src/arm/cache.c)
@@ -154,7 +160,7 @@ IF(CPUINFO_SUPPORTED_PLATFORM)
       ELSEIF(CMAKE_SYSTEM_PROCESSOR MATCHES "^(aarch64|arm64)$")
         LIST(APPEND CPUINFO_SRCS src/arm/linux/aarch64-isa.c)
       ENDIF()
-    ELSEIF(IOS OR (CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND CMAKE_SYSTEM_PROCESSOR STREQUAL "arm64"))
+    ELSEIF(IOS OR (CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND CPUINFO_TARGET_PROCESSOR STREQUAL "arm64"))
       LIST(APPEND CPUINFO_SRCS src/arm/mach/init.c)
     ENDIF()
     IF(CMAKE_SYSTEM_NAME STREQUAL "Android")
@@ -295,7 +301,7 @@ ENDIF()
 # ---[ cpuinfo mock library and mock tests
 IF(CPUINFO_SUPPORTED_PLATFORM AND CPUINFO_BUILD_MOCK_TESTS)
   SET(CPUINFO_MOCK_SRCS "${CPUINFO_SRCS}")
-  IF(CMAKE_SYSTEM_PROCESSOR MATCHES "^(i[3-6]86|AMD64|x86_64)$")
+  IF(CPUINFO_TARGET_PROCESSOR MATCHES "^(i[3-6]86|AMD64|x86_64)$")
     LIST(APPEND CPUINFO_MOCK_SRCS src/x86/mockcpuid.c)
   ENDIF()
   IF(CMAKE_SYSTEM_NAME STREQUAL "Linux" OR CMAKE_SYSTEM_NAME STREQUAL "Android")
@@ -739,7 +745,7 @@ IF(CPUINFO_SUPPORTED_PLATFORM AND CPUINFO_BUILD_UNIT_TESTS)
     ADD_TEST(get-current-test get-current-test)
   ENDIF()
 
-  IF(CMAKE_SYSTEM_PROCESSOR MATCHES "^(i[3-6]86|AMD64|x86_64)$")
+  IF(CPUINFO_TARGET_PROCESSOR MATCHES "^(i[3-6]86|AMD64|x86_64)$")
     ADD_EXECUTABLE(brand-string-test test/name/brand-string.cc)
     CPUINFO_TARGET_ENABLE_CXX11(brand-string-test)
     CPUINFO_TARGET_RUNTIME_LIBRARY(brand-string-test)
@@ -806,7 +812,7 @@ IF(CPUINFO_SUPPORTED_PLATFORM AND CPUINFO_BUILD_TOOLS)
     CPUINFO_TARGET_RUNTIME_LIBRARY(cpuinfo-dump)
   ENDIF()
 
-  IF(CMAKE_SYSTEM_PROCESSOR MATCHES "^(i[3-6]86|AMD64|x86_64)$")
+  IF(CPUINFO_TARGET_PROCESSOR MATCHES "^(i[3-6]86|AMD64|x86_64)$")
     ADD_EXECUTABLE(cpuid-dump tools/cpuid-dump.c)
     CPUINFO_TARGET_ENABLE_C99(cpuid-dump)
     CPUINFO_TARGET_RUNTIME_LIBRARY(cpuid-dump)