net-im/libsignal-client: Fix build on aarch64

aarch64 still uses unstable features:
error[E0554]: #![feature] may not be used on the stable release channel

--> /usr/ports/net-im/libsignal-client/work/libsignal-client-0.9.6/cargo-crates/polyval-0.5.1/src/lib.rs:80:5

80 | feature(stdsimd, aarch64_target_feature)

--> /usr/ports/net-im/libsignal-client/work/libsignal-client-0.9.6/cargo-crates/aes-0.7.4/src/lib.rs:87:5
7 | feature(stdsimd, aarch64_target_feature)

PR: 258371
Approved by: 0mp (maintainer)