libunwind: document tested FreeBSD configs and sort OS list

[emaste]

Summary

libunwind is known to work on FreeBSD/amd64 and FreeBSD/arm64, and is the default unwinder on both of those architectures.

While here sort the OS list.

[krytarowski]

@joerg might have insight on ppc, sparc64, arm on NetBSD.

[emaste]

In D38900#897304, @krytarowski wrote:

@joerg might have insight on ppc, sparc64, arm on NetBSD.

And @bsdjhb for other architectures on FreeBSD.

But I wanted to at least list x86_64 and ARM64 for FreeBSD, where we now have LLVM's libunwind in the base system enabled for those architectures.

[bsdjhb]

FreeBSD/i386 also uses llvm libunwind by default.

[mstorsjo]

Seems ok to me, with whatever other architectures it's verified on as well.

docs/index.rst

 Platform and Compiler Support
 -----------------------------
 
 libunwind is known to work on the following platforms:
 
 ============ ==================== ============ ========================
 OS           Arch                 Compilers    Unwind Info
 ============ ==================== ============ ========================
-Mac OS X     i386, x86_64         Clang, GCC   DWARF CFI
+Any          i386, x86_64, ARM    Clang        SjLj
+Bare Metal   ARM                  Clang, GCC   EHABI
+FreeBSD      x86_64, ARM64        Clang        DWARF CFI
 iOS          ARM                  Clang        SjLj
-Linux        i386, x86_64, ARM64  Clang, GCC   DWARF CFI
 Linux        ARM                  Clang, GCC   EHABI
-Bare Metal   ARM                  Clang, GCC   EHABI
+Linux        i386, x86_64, ARM64  Clang, GCC   DWARF CFI
+Mac OS X     i386, x86_64         Clang, GCC   DWARF CFI
 NetBSD       x86_64               Clang, GCC   DWARF CFI
-Any          i386, x86_64, ARM    Clang        SjLj
 ============ ==================== ============ ========================
 
 The following minimum compiler versions are strongly recommended.
 
 * Clang 3.5 and above
 * GCC 4.7 and above.
 
 Anything older *may* work.