macOS: Offer control over host architecture on Apple Silicon hosts

Since commit b6c60f14b6 (macOS: Default to arm64 architecture on Apple
Silicon hosts, 2020-09-28, v3.19.0-rc1~63^2) we use `sysctl` to detect
that we are running on Apple Silicon in a way that pierces Rosetta.
This always sets `CMAKE_HOST_SYSTEM_PROCESSOR` to be `arm64` on such
hosts.  However, macOS offers strong support for running processes under
an emulated `x86_64` architecture.

Teach CMake to select either `arm64` or `x86_64` as the host
architecture on Apple Silicon based on the architecture of its own
process.  When CMake is built as a universal binary, macOS will select
whichever slice (architecture) is appropriate under the user's shell,
and `CMAKE_HOST_SYSTEM_PROCESSOR` will match.

Also offer a `CMAKE_APPLE_SILICON_PROCESSOR` variable and environment
variable to provide users with explicit control over the host
architecture selection regardless of CMake's own architecture.

Finally, if `CMAKE_OSX_ARCHITECTURES` is not set, pass explicit flags to
the toolchain to use selected host architecture instead of letting the
toolchain pick.

Fixes: #21554

2 files changed, 45 insertions, 3 deletions

diff --git a/Help/variable/CMAKE_APPLE_SILICON_PROCESSOR.rst b/Help/variable/CMAKE_APPLE_SILICON_PROCESSOR.rst
new file mode 100644
index 0000000..ad297c3
--- /dev/null
+++ b/Help/variable/CMAKE_APPLE_SILICON_PROCESSOR.rst
@@ -0,0 +1,15 @@
+CMAKE_APPLE_SILICON_PROCESSOR
+-----------------------------
+
+.. versionadded:: 3.19.2
+
+On Apple Silicon hosts running macOS, set this variable to tell
+CMake what architecture to use for :variable:`CMAKE_HOST_SYSTEM_PROCESSOR`.
+The value must be either ``arm64`` or ``x86_64``.
+
+The value of this variable should never be modified by project code.
+It is meant to be set by a toolchain file specified by the
+:variable:`CMAKE_TOOLCHAIN_FILE` variable, or as a cache entry
+provided by the user, e.g. via ``-DCMAKE_APPLE_SILICON_PROCESSOR=...``.
+
+See also the :envvar:`CMAKE_APPLE_SILICON_PROCESSOR` environment variable.
diff --git a/Help/variable/CMAKE_HOST_SYSTEM_PROCESSOR.rst b/Help/variable/CMAKE_HOST_SYSTEM_PROCESSOR.rst
index 5f08728..c305779 100644
--- a/Help/variable/CMAKE_HOST_SYSTEM_PROCESSOR.rst
+++ b/Help/variable/CMAKE_HOST_SYSTEM_PROCESSOR.rst
@@ -3,13 +3,40 @@ CMAKE_HOST_SYSTEM_PROCESSOR
 
 The name of the CPU CMake is running on.
 
+Windows Platforms
+^^^^^^^^^^^^^^^^^
+
 On Windows, this variable is set to the value of the environment variable
-``PROCESSOR_ARCHITECTURE``. On systems that support ``uname``, this variable is
-set to the output of:
+``PROCESSOR_ARCHITECTURE``.
+
+Unix Platforms
+^^^^^^^^^^^^^^
+
+On systems that support ``uname``, this variable is set to the output of:
 
-- ``uname -m`` on GNU, Linux, Cygwin, Darwin, Android, or
+- ``uname -m`` on GNU, Linux, Cygwin, Android, or
 - ``arch`` on OpenBSD, or
 - on other systems,
 
   * ``uname -p`` if its exit code is nonzero, or
   * ``uname -m`` otherwise.
+
+macOS Platforms
+^^^^^^^^^^^^^^^
+
+The value of ``uname -m`` is used by default.
+
+On Apple Silicon hosts, the architecture printed by ``uname -m`` may vary
+based on CMake's own architecture and that of the invoking process tree.
+
+.. versionadded:: 3.19.2
+
+  On Apple Silicon hosts:
+
+  * The :variable:`CMAKE_APPLE_SILICON_PROCESSOR` variable or
+    the :envvar:`CMAKE_APPLE_SILICON_PROCESSOR` environment variable
+    may be set to specify the host architecture explicitly.
+
+  * If :variable:`CMAKE_OSX_ARCHITECTURES` is not set, CMake adds explicit
+    flags to tell the compiler to build for the host architecture so the
+    toolchain does not have to guess based on the process tree's architecture.