gst-plugins-rs: Address issues regarding iOS and Android

From !915 (merged) and !1191 (merged), the following bits were missing:

• Autotools expects the raw linker to perform detection for version scripting. If that parameter isn't detected in the --help, shared libraries are disabled (breaks libpng).
• Cargo, conversely, expects implicitly the clang executable itself because of a new dependency on libunwind starting with 1.68

The following changes were made:

• Passing the corresponding linkers separately, then LDFLAGS are massaged into link-args entries in the TOML's rustflags array.
• Enabled the debuginfo splitting to shave off a third of the size of the static plugins.
  • Postprocess the static libraries with objdump + strip to remove the remaining overhead.
• Blocked parallelism on Darwin platforms to prevent Cargo from starving the rest of the system.

Fixes #446 (closed)

Requires !1265 (merged) merged beforehand.