This target triple was weird on multiple levels:
* The `ilp32` ABI is the soft float ABI. This is not the main ABI we want to
support on RISC-V; rather, we want `ilp32d`.
* `gnuilp32` is a bespoke tag that was introduced in Zig. The rest of the world
just uses `gnu` for RISC-V target triples.
* `gnu_ilp32` is already the name of an ILP32 ABI used on AArch64. `gnuilp32` is
too easy to confuse with this.
* We don't use this convention for `riscv64-linux-gnu`.
* Supporting all RISC-V ABIs with this convention will result in combinatorial
explosion; see #20690.
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