Fix openmp CMake build on non-Linux AArch64 systems

[chandlerc]

Summary

Checks for /proc/cpuinfo before reading it.

[tianshilei1992]

LGTM. Thanks for the catch and fix!

[chandlerc]

Thanks for the fast review, landed in https://github.com/llvm/llvm-project/commit/f855751c1284c82c1c46b98f6d1b3ca2021d6cb9.

openmp/runtime/cmake/LibompGetArchitecture.cmake

function(libomp_get_architecture return_arch)
   # set the return value to the architecture detected (e.g., 32e, 32, arm, ppc64, etc.)
   set(${return_arch} "${local_architecture}" PARENT_SCOPE)
 
   # Remove ${detect_arch_src_txt} from cmake/ subdirectory
   file(REMOVE "${CMAKE_CURRENT_BINARY_DIR}/libomp_detect_arch.c")
 endfunction()
 
 function(libomp_is_aarch64_a64fx return_is_aarch64_a64fx)
+  set(is_aarch64_a64fx FALSE)
+  if (EXISTS "/proc/cpuinfo")
     file(READ "/proc/cpuinfo" cpu_info_content)
     string(REGEX MATCH "CPU implementer[ \t]*: 0x46\n" cpu_implementer ${cpu_info_content})
     string(REGEX MATCH "CPU architecture[ \t]*: 8\n" cpu_architecture ${cpu_info_content})
 
-  set(is_aarch64_a64fx FALSE)
     if (cpu_architecture AND cpu_implementer)
       set(is_aarch64_a64fx TRUE)
     endif()
+  endif()
 
   set(${return_is_aarch64_a64fx} "${is_aarch64_a64fx}" PARENT_SCOPE)
 endfunction(libomp_is_aarch64_a64fx)