well nope you're not the first and not the last but you have overlooked this:
LibreELEC-RR/arch.x86_64 at master-rr · SupervisedThinking/LibreELEC-RR · GitHub
which is only a fallback if this is not set:
LibreELEC-RR/options at master-rr · SupervisedThinking/LibreELEC-RR · GitHub
About the error... well actually I guess this is related to the host system which is probably bleeding edge at any package. It uses Kernel 5.11.y & GCC11 which is not even released yet?
Check out the cases of the distribution in:
LibreELEC-RR/checkdeps at libreelec-10.0-RR · SupervisedThinking/LibreELEC-RR · GitHub
these are the supported build host systems. Beside that NSS is a LE base package which isn't altered in any way by me so it's highly unlikely that it's failing because of changes I made.