[Docs] Modernize references to macOS

Summary:
This updates all places in documentation that refer to "Mac OS X", "OS X", etc.
to instead use the modern name "macOS" when no specific version number is
mentioned.

If a specific version is mentioned, this attempts to use the OS name at the time
of that version:

* Mac OS X for 10.0 - 10.7
* OS X for 10.8 - 10.11
* macOS for 10.12 - present

Reviewers: JDevlieghere

Subscribers: mgorny, christof, arphaman, cfe-commits, lldb-commits, libcxx-commits, llvm-commits

Tags: #clang, #lldb, #libc, #llvm

Differential Revision: https://reviews.llvm.org/D62654

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@362113 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/index.rst b/docs/index.rst
index 9e53b41..a4e21bb 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -50,7 +50,7 @@
 iOS          ARM                      Clang        SjLj
 Linux        ARM                      Clang, GCC   EHABI
 Linux        i386, x86_64, ARM64      Clang, GCC   DWARF CFI
-Mac OS X     i386, x86_64             Clang, GCC   DWARF CFI
+macOS        i386, x86_64             Clang, GCC   DWARF CFI
 NetBSD       x86_64                   Clang, GCC   DWARF CFI
 Windows      i386, x86_64, ARM, ARM64 Clang        DWARF CFI
 ============ ======================== ============ ========================