www/firefox: unbreak on armv6/armv7/powerpc64

error: Could not compile libc.
warning: build failed, waiting for other jobs to finish...
error[E0433]: failed to resolve: use of undeclared type or module mem

--> third_party/rust/libc/src/unix/bsd/freebsdlike/freebsd/powerpc64.rs:35:32
 |

35 | pub const _ALIGNBYTES: usize = mem::size_of::<::c_long>() - 1;

|                                ^^^ use of undeclared type or module `mem`

Reported by: pkg-fallout