Libc isn't the language runtime. The runtime is '/use/lib/crt*.o', which has no concept at all of signal handling.
Libc isn't particularly intrinsic to the language, and outside of some assembly to make syscalls, you can implement an alternative with a completely different interface, purely in C.
The language standard library does, however, contain explicit support for signal handling, as specified in ISO/IEC 1989:2023 section 7.14 Signal handling <signal.h>. The cross-platform bits of libc are specified in the C standard. The POSIX-specific bits are specified by the Open Group in the POSIX standard. The OS-specific bits are specified by the OS and implemented by whoever is writing the libc in question. A libc is a sort of statically linked combination of the C standard library and some OS-specific standard library extensions.
I am fairly certain that glibc uses SA_RESTORER in its sigaction wrapper and implements a suitable sigreturn() function which is provided as the sa_restorer argument.
