`riscv64-unknown-freebsd` patches backtrace support out

[estebank]

https://github.com/freebsd/freebsd-ports/blob/main/lang/rust/files/patch-library_backtrace_src_backtrace_mod.rs

Since late 2021.

[bjorn3]

The patch you linked is for riscv64, but the commit you linked disabled it for arm. freebsd/freebsd-ports@53364ad is the commit that disabled it for riscv64.