Eliminate include of system signal.h from core/ builds

We've had various define conflicts and problems in the past from the system signal.h conflicting with kernel-level or other defines we need inside DR. Today we still include signal.h, but we have our own distinctly-named types like `kernel_sigcontext_t` to avoid conflicts. But we still have some shared types, including on aarch64 where `struct fpsimd_context` ends up coming from the system includes. Xref PR #4325 where I added it to our own sigcontext.h to support targeting aarch64 on x86, but it would better to rename the type, and even better to eliminate the signal.h include. Xref the discussion at https://github.com/DynamoRIO/dynamorio/pull/4325#discussion_r446257978