backends/: fixup x86 initreg_sample to share code with aarch64

backends/: fixup x86 initreg_sample to share code with aarch64

A bunch of x86 sample_regs code should be common across
architectures. In particular, sample_sp_pc() implementation looks to
be universally applicable apart from the register positions.

* backends/x86_initreg_sample.c (x86_sample_sp_pc): Remove in favour
  of generic_sample_sp_pc (added to libebl_PERF_FLAGS.h in prev patch).
  (x86_sample_perf_regs_mapping): This should have been an inline
  function. Fix.
* backends/i386_init.c (i386_init): Use
  __libebl_init_cached_regs_mapping.
* backends/x86_64_init.c (x86_64_init): Use
  __libebl_init_cached_regs_mapping.
* backends/i386_initreg_sample.c (i386_sample_sp_pc): Use
  generic_sample_sp_pc.
* backends/x86_64_init.c (x86_64_initreg_sample): Use
  generic_sample_sp_pc.

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