Enable PyTorch QNNPACK crosscompilation

Introduce changes similar to pytorch/cpuinfo#51 to detect that PyTorch is build on MacOS  for different architecture

Test Plan:
  Pull shibatch/sleef#376 and run `cmake -DCMAKE_OSX_ARCHITECTURES=x86_64 -DPYTHON_EXECUTABLE=/usr/bin/python3 -DUSE_NNPACK=NO -DUSE_XNNPACK=NO -DBUILD_TEST=YES .. -G Ninja; ninja basic`

aten/src/ATen/native/quantized/cpu/qnnpack/CMakeLists.txt

@@ -21,6 +21,12 @@ option(PYTORCH_QNNPACK_BUILD_BENCHMARKS "Build QNNPACK benchmarks" ON)
 # Enable runtime requantization.
 add_definitions(-DPYTORCH_QNNPACK_RUNTIME_QUANTIZATION=1)
 
+# ---[ Target processor
+SET(PYTORCH_QNNPACK_TARGET_PROCESSOR "${CMAKE_SYSTEM_PROCESSOR}")
+IF(CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND CMAKE_OSX_ARCHITECTURES MATCHES "^(x86_64|arm64)$")
+  SET(PYTORCH_QNNPACK_TARGET_PROCESSOR "${CMAKE_OSX_ARCHITECTURES}")
+ENDIF()
+
 # ---[ CMake options
 if(PYTORCH_QNNPACK_BUILD_TESTS)
   enable_testing()
@@ -244,11 +250,11 @@ if(CMAKE_SYSTEM_PROCESSOR MATCHES "^armv[5-8]" OR IOS_ARCH MATCHES "^armv7")
   list(APPEND PYTORCH_QNNPACK_UKERNELS ${PYTORCH_QNNPACK_ARM_NEON_UKERNELS})
   list(APPEND PYTORCH_QNNPACK_UKERNELS ${PYTORCH_QNNPACK_AARCH32_ASM_UKERNELS})
 endif()
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "^(aarch64|arm64)$" OR IOS_ARCH MATCHES "^arm64.*")
+if(PYTORCH_QNNPACK_TARGET_PROCESSOR MATCHES "^(aarch64|arm64)$" OR IOS_ARCH MATCHES "^arm64.*")
   list(APPEND PYTORCH_QNNPACK_UKERNELS ${PYTORCH_QNNPACK_ARM_NEON_UKERNELS})
   list(APPEND PYTORCH_QNNPACK_UKERNELS ${PYTORCH_QNNPACK_AARCH64_ASM_UKERNELS})
 endif()
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "^(i[3-6]86|x86_64)$" OR IOS_ARCH MATCHES "^(i386|x86_64)$")
+if(PYTORCH_QNNPACK_TARGET_PROCESSOR MATCHES "^(i[3-6]86|x86_64)$" OR IOS_ARCH MATCHES "^(i386|x86_64)$")
   list(APPEND PYTORCH_QNNPACK_UKERNELS ${PYTORCH_QNNPACK_X86_SSE2_UKERNELS})
 endif()
 
@@ -271,13 +277,13 @@ if(CMAKE_SYSTEM_PROCESSOR MATCHES "^armv[5-8]" OR IOS_ARCH MATCHES "^armv7")
     set_property(SOURCE ${PYTORCH_QNNPACK_AARCH32_ASM_UKERNELS} APPEND_STRING PROPERTY COMPILE_FLAGS " -arch ${IOS_ARCH} ")
   endif()
 endif()
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "^(aarch64|arm64)$" OR IOS_ARCH MATCHES "^arm64.*")
+if(PYTORCH_QNNPACK_TARGET_PROCESSOR MATCHES "^(aarch64|arm64)$" OR IOS_ARCH MATCHES "^arm64.*")
   set_property(SOURCE ${PYTORCH_QNNPACK_ARM_NEON_UKERNELS} APPEND_STRING PROPERTY COMPILE_FLAGS " -O2 ")
   if(IOS)
     set_property(SOURCE ${PYTORCH_QNNPACK_AARCH64_ASM_UKERNELS} APPEND_STRING PROPERTY COMPILE_FLAGS " -arch ${IOS_ARCH} ")
   endif()
 endif()
-if(CMAKE_SYSTEM_PROCESSOR MATCHES "^(i[3-6]86|x86_64)$" OR IOS_ARCH MATCHES "^(i386|x86_64)$")
+if(PYTORCH_QNNPACK_TARGET_PROCESSOR MATCHES "^(i[3-6]86|x86_64)$" OR IOS_ARCH MATCHES "^(i386|x86_64)$")
   set_property(SOURCE ${PYTORCH_QNNPACK_X86_SSE2_UKERNELS} APPEND_STRING PROPERTY COMPILE_FLAGS " -O2 -msse2 ")
 endif()
 if(CMAKE_SYSTEM_PROCESSOR MATCHES "^armv[5-8]" OR IOS_ARCH MATCHES "^armv7")