allow building for iOS 32-bit

[kambala-decapitator]

Description

Building for iOS armv7/armv7s fails with the following error:

ninja: error: '../src/tbb/def/mac32-tbb.def', needed by 'appleclang_13.1_cxx11_32_release/libtbb.12.7.dylib', missing and no known rule to make it

This change fixes it.

Tested by building with Ninja and Makefiles generators using ios-cmake toolchain, also built the Getting Started example.

cmake -S . -B build -G Ninja \
  -DTBB_TEST=OFF \
  -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_TOOLCHAIN_FILE=~/dev/vcmi/vcmi-ios-depends/ios-cmake/ios.toolchain.cmake \
  -DPLATFORM=OS \
  -DENABLE_BITCODE=OFF \
  -DDEPLOYMENT_TARGET=10.0 \
  -DENABLE_ARC=ON \
  -DENABLE_VISIBILITY=ON

• - git commit message contains an appropriate signed-off-by string (see CONTRIBUTING.md for details)

Type of change

• bug fix - change that fixes an issue
• new feature - change that adds functionality
• tests - change in tests
• infrastructure - change in infrastructure and CI
• documentation - documentation update

Tests

• added - required for new features and some bug fixes
• not needed

Documentation

• updated in # - add PR number
• needs to be updated
• not needed

Breaks backward compatibility

• Yes
• No
• Unknown

Other information

[pavelkumbrasev]

Hi @kambala-decapitator, how it would link without def file? What the propose of this patch?

[kambala-decapitator]

@pavelkumbrasev

how it would link without def file?

I've never used def files before, but to my understanding, if you don't provide it, then linker simply takes all symbols it can find.

What the propose of this patch?

sorry, thought it's clear from PR title: to make TBB usable on 32-bit (armv7) iOS devices.

Here's an example of built library with this patch (fat binary with armv7 and arm64), if you'd like to see what was linked in it:
libtbb.12.5.dylib.tar.gz