Revert vita's c_char back to i8
# Description
Hi!
https://github.com/rust-lang/rust/pull/132975 changed the definition of `c_char` from i8 to u8 for most ARM targets. While that would usually be correct, [VITASDK uses signed chars by default](https://github.com/vitasdk/buildscripts/blob/master/patches/gcc/0001-gcc-10.patch#L33-L34). The Clang definitions are incorrect because Clang is not (yet?) supported by the vita commmunity / `VITADSK`, On the Rust side, the pre-compiled libraries the user can link to are all compiled using vita's `gcc` and [we set `TARGET_CC` and `TARGET_CXX`](d564a132cb/src/commands/build.rs (L230)) in `cargo vita` for build scripts using `cc`.
I'm creating it as a draft PR so that we can discuss it and possibly get it approved here, but wait to merge the [libc side](https://github.com/rust-lang/libc/pull/4258) and get a libc version first, as having the definitions out of sync breaks std. As a nightly-only target it can be confusing/frustrating for new users when the latest nightly, which is the default, is broken.
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