Set use-jemalloc conditionally instead of ignoring it for some targets

src/main.rs and Cargo.toml include a complex expression to not use jemalloc on some targets even when use-jemalloc has been set explicitly. As far as I can tell most of the cases where jemalloc is disabled is due to build failures. Since cargo can be configured to use a system provided jemalloc these build failures can be avoided on some targets (in my case on 32-bit musl and riscv64). Therefor it would be nice to conditionally have use-jemalloc in the default features instead of force disabling it on some targets.