Rollup merge of #149557 - folkertdev:c-variadic-not-supported, r=workingjubilee

c-variadic: bpf and spirv do not support c-variadic definitions

tracking issue: https://github.com/rust-lang/rust/issues/44930

Emit a nice error message on bpf and spirv targets when a c-variadic function is defined. Spirv also does not support c-variadic calls, bpf appears to allow them.

r? ```@workingjubilee```