Update macOS support level and document how to dequarantine the tools (#227)

On macOS when the files are unpacked the com.apple.quarantine will be
added to the executable files. Document that user is responsible for
removing the attribute and give the user a command to run to do this.

This issue will be addressed in a follow up change.

Author: smithp35

README.md

@@ -59,8 +59,7 @@ LLVM Embedded Toolchain for Arm is built and tested on Ubuntu 18.04 LTS.
 
 The Windows version is built on Windows Server 2019 and lightly tested on Windows 10.
 
-Building on macOS is functional but experimental. Currently it is only lightly tested on
-a Macbook Pro with M1 on macOS 12.3.1.
+Building on macOS is functional for x86_64 and Apple Silicon.
 
 [Binary packages](https://github.com/ARM-software/LLVM-embedded-toolchain-for-Arm/releases)
 are provided for major LLVM releases for Linux and Windows.
@@ -73,6 +72,14 @@ and extract the archive into an arbitrary directory.
 
 On Ubuntu 20.04 and later `libtinfo5` is required: `apt install libtinfo5`.
 
+On macOS the toolchain binaries are quarantined by com.apple.quarantine. To
+run the executables change directory to bin and run the following command to
+remove the com.apple.quarantine:
+
+```
+find . -type f -perm +0111 | xargs xattr -d com.apple.com.quarantine
+```
+
 ### Using the toolchain
 
 To use the toolchain, on the command line you need to provide: