riscv: export riscv_sp_ptr

riscv: export riscv_sp_ptr

Export SP register for later use by get_rt_sigframe_addr.

* linux/riscv/arch_regs.h: New file.
* Makefile.am (EXTRA_DIST): Add it.
* linux/riscv/arch_regs.c (riscv_sp_ptr): New variable.