fix: properly skip fp16kernels if not enabled and allow building on ios

[chyyran]

Previously even if fp16kernels was disabled, the check would still fail and cause a build failure when trying to build lance-linalg.

The motivation was to enable building on aarch64-apple-ios. This PR also adds support for mtune=apple-a13 when building for iOS, which is the earliest supported iOS architecture at time of writing.

This is intended for merge to the 1.0 branch of lance. Let me know if its preferable to PR it to main and backport the change after.

[wjones127]

A similar change was previously suggested in #5747

However, I pointed out we might still want to run build_dist_table_with_flags even if we haven't enabled fp16kernels. We already guard before building the kernels here:

lance/rust/lance-linalg/build.rs

Lines 92 to 97 in 303f58f

	if cfg!(not(feature = "fp16kernels")) {
	println!(
	"cargo:warning=fp16kernels feature is not enabled, skipping build of fp16 kernels"
	);
	return Ok(());
	}

What was the build failure you were getting?

[wjones127]

See previous comment.

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[chyyran]

@wjones127 When building the main branch of LanceDB (lance-linalg 2.0.0 last I checked)

error: failed to run custom build command for `lance-linalg v2.0.0`

Caused by:
  process didn't exit successfully: `/Users/ronny/coding/lena/lancedb/target/debug/build/lance-linalg-0bf9b9dda1273bf3/build-script-build` (exit status: 1)
  --- stdout
  cargo::rustc-check-cfg=cfg(kernel_support, values("avx512"))
  cargo:rerun-if-changed=src/simd/f16.c
  cargo:rerun-if-changed=src/simd/dist_table.c

  --- stderr
  Error: "Unable to build f16 kernels on given target_arch.  Please use x86_64 or aarch64 or remove the fp16kernels feature"
warning: build failed, waiting for other jobs to finish...

After #5747 the build error seems to be fixed. I can't rebase this on main so I'll open a new PR for the A13 optimized build on iOS.