backends/: should use size_t n_regs_mapping for *_sample_sp_pc

backends/: should use size_t n_regs_mapping for *_sample_sp_pc

These should have been size_t all along, matching the public
libdwfl_stacktrace dwflst_sample_getframes interface. Fix.

* backends/aarch64_initreg_sample.c (aarch64_sample_sp_pc): Fixup
  argument type for n_regs_mapping.
* backends/i386_initreg_sample.c (i386_sample_sp_pc): Fixup argument
  type for n_regs_mapping.
* backends/libebl_PERF_FLAGS.h (generic_sample_sp_pc): Fixup argument
  type for n_regs_mapping, adjust loop index var accordingly.
* backends/x86_64_initreg_sample.c (x86_64_sample_sp_pc): Fixup
  argument type for n_regs_mapping.
* libebl/ebl-hooks.h (sample_sp_pc): Fixup argument type for
  n_regs_mapping.

Signed-off-by: Serhei Makarov <serhei@serhei.io>