www/firefox{,-esr}, mail/thunderbird: remote LTO from default options

When rust's internal LLVM does not match the LLVM used for building
gecko ports, LTO-built binaries will be unstable, exhibit crashes
and other undesirable behaviour. Rust 1.60 created such a situation.
Disabling LTO will allow these ports to be used, and keeping it off
will safe on build and debug time.

(cherry picked from commit fc0a979a30ea24c8d28d8a3893bb5ac0549e56cb)