[libcxx] Fix the definition of the check-cxx-abilist target on Darwin

Summary:
r342805 added support for the check-cxx-abilist target on FreeBSD, but broke
the target on macOS in doing so. The problem is that the GENERIC_TARGET_TRIPLE
gets overwritten after replacing the FreeBSD regular expression, which
nullifies the replacement done with the darwin regular expression.

Reviewers: dim, EricWF

Subscribers: emaste, mgorny, krytarowski, christof, dexonsmith, cfe-commits, libcxx-commits

Differential Revision: https://reviews.llvm.org/D52394

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@342813 91177308-0d34-0410-b5e6-96231b3b80d8

lib/abi/CMakeLists.txt

@@ -1,12 +1,16 @@
 
 if (DEFINED TARGET_TRIPLE)
-    # Ignore the major, minor, and patchlevel versions of darwin targets.
-    string(REGEX REPLACE "darwin[0-9]+\\.[0-9]+\\.[0-9]+" "darwin"
-           GENERIC_TARGET_TRIPLE "${TARGET_TRIPLE}")
-
-    # Ignore the major and minor versions of freebsd targets.
-    string(REGEX REPLACE "freebsd[0-9]+\\.[0-9]+" "freebsd"
-           GENERIC_TARGET_TRIPLE "${TARGET_TRIPLE}")
+    if (TARGET_TRIPLE MATCHES "darwin")
+        # Ignore the major, minor, and patchlevel versions of darwin targets.
+        string(REGEX REPLACE "darwin[0-9]+\\.[0-9]+\\.[0-9]+" "darwin"
+               GENERIC_TARGET_TRIPLE "${TARGET_TRIPLE}")
+    elseif(TARGET_TRIPLE MATCHES "freebsd")
+        # Ignore the major and minor versions of freebsd targets.
+        string(REGEX REPLACE "freebsd[0-9]+\\.[0-9]+" "freebsd"
+               GENERIC_TARGET_TRIPLE "${TARGET_TRIPLE}")
+    else()
+        set(GENERIC_TARGET_TRIPLE "${TARGET_TRIPLE}")
+    endif()
 endif()
 
 # Detect if we are building in the same configuration used to generate