Enable cross-language inlining for sleef-sys

[gnzlbg]

See rust-lang/rust#53031 (comment)

• compile you're C code with clang 7 and -flto=thin,
• make sure that you are using LLD as your linker (e.g. with -Clinker=clang -Clink-arg=-fuse-ld=lld,
• add -Zcross-lang-lto -Ccodegen-units=1 to your RUSTFLAGS.