[libc][cmake] Fix LIBC_TARGET_OS query from compiler triple for `darwin`.

[lntue]

Summary

Currently LIBC_TARGET_OS query from compiler triple returns apple for
macOS. This change will fix it back to darwin as before.

libc/cmake/modules/LLVMLibCArchitectures.cmake

function(get_arch_and_system_from_triple triple arch_var sys_var)
   math(EXPR system_index "${triple_size} - 2")
   list(GET triple_comps 0 target_arch)
   # The target_arch string can have sub-architecture suffixes which we want to
   # remove. So, we regex-match the string and set target_arch to a cleaner
   # value.
   if(target_arch MATCHES "^mips")
     set(target_arch "mips")
   elseif(target_arch MATCHES "^arm")
+    # TODO(lntue): Shall we separate `arm64`?  It is currently recognized as
+    # `arm` here.
     set(target_arch "arm")
   elseif(target_arch MATCHES "^aarch64")
     set(target_arch "aarch64")
   elseif(target_arch MATCHES "(x86_64)|(AMD64|amd64)|(^i.86$)")
     set(target_arch "x86_64")
   elseif(target_arch MATCHES "^(powerpc|ppc)")
     set(target_arch "power")
   else()
     return()
   endif()
 
   set(${arch_var} ${target_arch} PARENT_SCOPE)
   list(GET triple_comps ${system_index} target_sys)
+
+  # Correcting OS name for Apple's systems.
+  if(target_sys STREQUAL "apple")
+    list(GET triple_comps 2 target_sys)
+  endif()
+  # Strip version from `darwin###`
+  if(target_sys MATCHES "^darwin")
+    set(target_sys "darwin")
+  endif()
+
   set(${sys_var} ${target_sys} PARENT_SCOPE)
 endfunction(get_arch_and_system_from_triple)
 
 # Query the default target triple of the compiler.
 set(target_triple_option "-print-target-triple")
 if(CMAKE_COMPILER_IS_GNUCXX)
   # GCC does not support the "-print-target-triple" option but supports
   # "-print-multiarch" which clang does not support for all targets.