build: bump executorch to v1.2.0

[barhanc]

Description

Updates ExecuTorch binaries and header files to version v1.2.0

Introduces a breaking change?

• Yes
• No

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

• Run tests from packages/react-native-executorch/common/rnexecutorch/tests.
• Build all example apps on both Android and iOS and verify that models work as expected.

Screenshots

Related issues

Closes #1057

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings

Additional notes

The binaries were built from https://github.com/barhanc/executorch/tree/rne-build-v1.2.0 according to instructions here. The header files also come from there.