[llvm][Docs] Update supported hardware

Add a few known platforms:
* Linux AArch64
* FreeBSD AArch64
* macOS arm64 (Clang build only, there might be a GCC port but I've not used it myself)
* Windows on Arm (ARM64 as Microsoft refers to it)
  (you could say this builds with Clang too, but you need a Visual Studio install
  as well, so I don't want to compilicate things by mentioning Clang here)

Author: DavidSpickett

llvm/docs/GettingStarted.rst

@@ -239,22 +239,26 @@ OS                 Arch                  Compilers
 Linux              x86\ :sup:`1`         GCC, Clang
 Linux              amd64                 GCC, Clang
 Linux              ARM                   GCC, Clang
+Linux              AArch64               GCC, Clang
 Linux              Mips                  GCC, Clang
 Linux              PowerPC               GCC, Clang
 Linux              SystemZ               GCC, Clang
 Solaris            V9 (Ultrasparc)       GCC
 DragonFlyBSD       amd64                 GCC, Clang
 FreeBSD            x86\ :sup:`1`         GCC, Clang
 FreeBSD            amd64                 GCC, Clang
+FreeBSD            AArch64               GCC, Clang
 NetBSD             x86\ :sup:`1`         GCC, Clang
 NetBSD             amd64                 GCC, Clang
 OpenBSD            x86\ :sup:`1`         GCC, Clang
 OpenBSD            amd64                 GCC, Clang
 macOS\ :sup:`2`    PowerPC               GCC
 macOS              x86                   GCC, Clang
+macOS              arm64                 Clang
 Cygwin/Win32       x86\ :sup:`1, 3`      GCC
 Windows            x86\ :sup:`1`         Visual Studio
 Windows x64        x86-64                Visual Studio
+Windows on Arm     ARM64                 Visual Studio
 ================== ===================== =============
 
 .. note::