purego: fix ARM64 struct register corruption on alignment flush

Fix a critical bug in ARM64 struct argument packing where the register
value (val) and class were not being reset after flushing due to
alignment requirements.

When packing struct fields into registers, if a field's alignment
causes shift >= 64, the current register is flushed. However, the
code was not resetting 'val' and 'class' after the flush, causing
subsequent fields to be ORed with stale data from previous fields.

Example bug with FourInt32s{1, 2, 3, 4}:
- Fields 0,1 packed: val = 0x0000000200000001
- Flush at field 2 due to shift >= 64
- BUG: val still contains 0x0000000200000001
- Field 2 packs: val |= 3 becomes 0x0000000200000003 (should be 0x03)
- Field 3 packs: val |= (4<<32) becomes 0x0000000400000003
- Result: field 3 = 6 instead of 4 (bit 1 from field 1 leaked)

This fix ensures val and class are properly reset after each flush,
preventing data corruption between register boundaries.

Fixes the FourInt32s test case that was failing with f3=6 instead of f3=4.

Author: tmc

struct_arm64.go

@@ -117,6 +117,8 @@ func placeRegisters(v reflect.Value, addFloat func(uintptr), addInt func(uintptr
 				} else {
 					addInt(uintptr(val))
 				}
+				val = 0
+				class = _NO_CLASS
 			}
 			switch f.Type().Kind() {
 			case reflect.Struct: