Fix undefined pthread_ references on glibc < 2.34

[timsong1]

Summary

Building tools/rvsim with mini-gdbstub enabled can fail on systems with glibc < 2.34 due to missing pthread symbols such as pthread_create / pthread_join.
Because libgdbstub.a uses pthread APIs, but rvsim’s link flags do not include pthread, causing the final link step to fail.
On glibc >= 2.34, this issue may not trigger a link error because glibc merged libpthread functionality into libc

Change

Add pthread to the link flags in tools/Makefile:

- LDFLAGS += -Lmini-gdbstub/build -lgdbstub
+ LDFLAGS += -Lmini-gdbstub/build -lgdbstub -lpthread

Note

This change is required on glibc < 2.34 where pthread symbols are not provided by libc by default.
On glibc >= 2.34, -lpthread remains safe because glibc keeps the linker option working for backwards compatibility (empty libpthread.a is provided), so this should not affect newer systems.