c-call:char should have the same signedness as C char

Describe the bug

Currently c-call:char is a signed char. But to match C, c-call:char should have the same signedness as a C char. In particular, this means it should be unsigned for RISC-V, PowerPC, and ARM (among others). (See !157 (comment 12484)).

Expected behavior

c-call:char should match C char signedness.

Desktop (please complete the following information):

• OS: All
• Version: 21e (and earlier)