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

1 files changed, 13 insertions, 0 deletions

diff --git a/Help/envvar/CMAKE_APPLE_SILICON_PROCESSOR.rst b/Help/envvar/CMAKE_APPLE_SILICON_PROCESSOR.rst
new file mode 100644
index 0000000..adecb72
--- /dev/null
+++ b/Help/envvar/CMAKE_APPLE_SILICON_PROCESSOR.rst
@@ -0,0 +1,13 @@
+CMAKE_APPLE_SILICON_PROCESSOR
+-----------------------------
+
+.. versionadded:: 3.19.2
+
+.. include:: ENV_VAR.txt
+
+On Apple Silicon hosts running macOS, set this environment variable to tell
+CMake what architecture to use for :variable:`CMAKE_HOST_SYSTEM_PROCESSOR`.
+The value must be either ``arm64`` or ``x86_64``.
+
+The :variable:`CMAKE_APPLE_SILICON_PROCESSOR` normal variable, if set,
+overrides this environment variable.