Rollup merge of #151572 - androm3da:bcain/abi_cconv, r=wesleywiser

Fix Hexagon ABI calling convention for small aggregates

Small structs (<= 64 bits) were being passed with their fields split into separate arguments instead of being packed into register-sized chunks. This caused ABI mismatches.

The fix properly casts small aggregates to consecutive register-sized chunks using Uniform::consecutive(), matching the Hexagon C ABI where small structs are packed into R1:0 register pair.

This fixes tests like extern-pass-TwoU16s.rs and extern-pass-TwoU8s.rs.

Author: JonathanBrouwer