lang/rust: Cleanup part 4

• Spell LICENSE_FILE_APACHE20 correctly
• Move gmake to BUILD_DEPENDS directly. gmake is called during the build by some crates but is not the primary build tool.
• Move variables around to be more in line with the recommendations in the Porter's Handbook
• Mark port local non-overridable variables as "private"
• Reduce noise of RUST_ARCH_*: only keep the overrides when they differ from ${ARCH}
• Drop unused RUST_TARGET plist sub
• Move post-configure-DOCS-* into do-configure