Build failure on macOS 10.4 on arm64

[jberdine]

Presumably due to the same clang bug as caused janestreet/base#180, and probably the fix is the same from the looks of:

ocaml_intrinsics/src/discover/discover.ml

Lines 112 to 117 in eed738b

	(fun (flag, prog) ->
	match c_test c ~c_flags:[ flag ] prog with
	| true -> Some flag
	| false -> None)
	[ "-mpopcnt", prog_popcnt (* ; "-mlzcnt", prog_lzcnt
	* ; "-mbmi", prog_tzcnt *)

Build log for reference:

#=== ERROR while compiling ocaml_intrinsics.v0.16.1 ===========================#
# context     2.3.0 | macos/arm64 | ocaml-option-flambda.1 ocaml-option-nnp.1 ocaml-variants.4.14.2+options | https://opam.ocaml.org#ad2202b486885c0c13f146ead5c85646ce87e24e
# path        ~/.opam/default/.opam-switch/build/ocaml_intrinsics.v0.16.1
# command     ~/.opam/opam-init/hooks/sandbox.sh build dune build -p ocaml_intrinsics -j 15
# exit-code   1
# env-file    ~/.opam/log/ocaml_intrinsics-3609-d46b77.env
# output-file ~/.opam/log/ocaml_intrinsics-3609-d46b77.out
### output ###
# [...]
# File "src/dune", line 6, characters 4-18:
# 6 |     prefetch_stubs)
#         ^^^^^^^^^^^^^^
# (cd _build/default/src && /usr/bin/cc -O2 -fno-strict-aliasing -fwrapv -pthread -D_FILE_OFFSET_BITS=64 -O2 -fno-strict-aliasing -fwrapv -pthread -march=armv8-a+crc -mpopcnt -mprfchw -g -I /Users/josh/.opam/default/lib/ocaml -o prefetch_stubs.o -c prefetch_stubs.c)
# clang: error: unsupported option '-mpopcnt' for target 'arm64-apple-darwin24.4.0'
# clang: error: unsupported option '-mprfchw' for target 'arm64-apple-darwin24.4.0'
# File "src/dune", line 4, characters 44-57:
# 4 |   (names atomic_stubs int_stubs float_stubs perfmon_stubs crc_stubs
#                                                 ^^^^^^^^^^^^^
# (cd _build/default/src && /usr/bin/cc -O2 -fno-strict-aliasing -fwrapv -pthread -D_FILE_OFFSET_BITS=64 -O2 -fno-strict-aliasing -fwrapv -pthread -march=armv8-a+crc -mpopcnt -mprfchw -g -I /Users/josh/.opam/default/lib/ocaml -o perfmon_stubs.o -c perfmon_stubs.c)
# clang: error: unsupported option '-mpopcnt' for target 'arm64-apple-darwin24.4.0'
# clang: error: unsupported option '-mprfchw' for target 'arm64-apple-darwin24.4.0'

[dkalinichenko-js]

Thanks for the report! I submitted ocaml/opam-repository#27755, which should resolve the issue on the v0.16 branch.