test: Fix test/IRGen/stdlib/Mutex.swift for 32-bit platforms
The test was failing on 32-bit platforms because the offset of the
layout of struct metadata depends on the pointer size.
| Field | Type | Offset (32-bit) | Offset (64-bit) |
|-------------------------|------|-----------------|-----------------|
| Layout String Pointer | ptr | 4 * (-2) | 4 * (-4) |
| VWT | ptr | 4 * (-1) | 4 * (-2) |
| Metadata Flags | ptr | 4 * 0 | 4 * 0 |
| Nominal Type Descriptor | ptr | 4 * 1 | 4 * 2 |
| Generic Requirement | ptr | 4 * 2 | 4 * 4 |
| Field Offset for handle | i32 | 4 * 3 | 4 * 6 |
| Field Offset for value | i32 | 4 * 4 | 4 * 7 |
