Fix rustcflags mapping: require -Clinker-plugin-lto for -flto

[NobodyXu]

Only when -Clto and -Clinker-plugin-lto are both specified, would we pass -flto, making sure that we won't break environments which has ar doesn't support llvm ir bitcode and linker that doesn't have llvm lto plugin.

Fixed #1463

[dot-asm]

While you're at it

cc-rs/src/flags.rs

Line 516 in 9a2e7ce

"-Clinker-plugin-lto=yes",

is non-representative of reality.

[NobodyXu]

While you're at it

cc-rs/src/flags.rs

Line 516 in 9a2e7ce

"-Clinker-plugin-lto=yes",

is non-representative of reality.

Thanks, fixed