sizeof(ssize_t) is not equal to sizeof(size_t) for some 64-bit targets by clang

The code example: https://godbolt.org/z/Y5WWTYGza

-O1 --target=aarch64-unknown-linux-gnu
-O1 --target=mips64-unknown-linux-gnu
-O1 --target=riscv64-unknown-linux-gnu
-O1 --target=riscv64be-unknown-linux-gnu
-O1 --target=ppc64-unknown-linux-gnu
-O1 --target=ppc64le-unknown-linux-gnu

Rationale:

https://www.mail-archive.com/austin-group-l@opengroup.org/msg14371.html

The gcc for these target triple that can ensure sizeof(ssize_t) === sizeof(size_t)

Start by reproducing the Godbolt example with Clang at -O1 for the listed 64-bit target triples, then compare the resulting sizeof(ssize_t) and sizeof(size_t) with the stated GCC behavior. Read the Austin Group rationale and trace the relevant target-specific type definitions. Done means the sizes agree for the reported targets and regression coverage verifies them.

Written by the indexing model from the issue text.