Fix iOS FFmpeg toolchain selection for asm objects

[sjfhsjfh]

Summary:

• keep the iOS and iOS simulator toolchain commands in target config
• pass --cc, --cxx, --as, and --ld explicitly to FFmpeg configure for Apple mobile targets
• prevent configure from falling back to host gcc, which produced macOS asm objects in iOS archives

Root cause:

• on the recent macOS runner image, FFmpeg configure ended up with CC=gcc, AS=gcc, and LD=gcc in ffbuild/config.mak for aarch64-apple-ios
• C objects still used the iPhoneOS sysroot from CFLAGS, but asm objects such as float_dsp_neon.o were assembled as macOS objects
• explicitly wiring the toolchain into configure makes the asm objects build for iOS again

Validation:

• reproduced locally by inspecting ffbuild/config.mak and Mach-O load commands
• before this change, float_dsp_neon.o was platform 1 (macOS)
• with this change, float_dsp_neon.o becomes platform 2 (iOS)

Testing:

• local reproduction of python3 scripts/build-ffmpeg.py aarch64-apple-ios
• CI workflow_dispatch on this branch should regenerate the iOS artifacts

Attribution:

• prepared with Codex (OpenAI) under user direction