basilisk: fix rpath-link, mach package, pre-install for euxp build

- Add -Wl,-rpath-link,dist/bin to OS_LDFLAGS so GNU ld finds transitive
  NSS/sqlite deps via libxul.so DT_NEEDED when linking xpcshell etc.
- Replace MKDIRS with mkdir -p (MKDIRS undefined in OZSW GAR)
- Add mach package to pre-build with LD_LIBRARY_PATH=/opt/ozsw/lib
  (required so xpcshell can find libgtk-3.so.0 during startup cache precompile)
- Patch package-manifest.in for euxp flat layout:
  comment out content_geckomediaplugins.xpt (not built),
  fix devtools paths (no browser/ prefix, defaults/pref not defaults/preferences)
- pre-install: extract from Basilisk-*.tar.xz tarball, chmod Basilisk binaries
- Wrapper script uses Basilisk (capital B) not basilisk