libebl: api for perf register handling, start with x86_64

libebl: api for perf register handling, start with x86_64

As it happens, Linux perf_events and DWARF can prescribe completely
different layouts for the register file, requiring non-obvious code
for translation. This makes sense to put in libebl if we want
profilers to handle perf sample data with elfutils.

Start with the x86_64 implementation taken from eu-stacktrace. Needs
some additional testing as the code has been generalized to accept
other perf register masks besides the 'preferred' one.

* backends/Makefile.am (x86_64_SRCS): Add x86_64_initreg_sample.c.
* backends/libebl_PERF_FLAGS.h: New file.
* backends/x86_64_initreg_sample.c: New file, implements a generalized
  version of the eu-stacktrace register shuffling for x86_64.
* backends/x86_64_init.c (x86_64_init): Add initialization for
  set_initial_registers_sample, perf_frame_regs_mask.
* libebl/Makefile.am (libebl_a_SOURCES): Add eblinitreg_sample.c.
* libebl/ebl-hooks.h (set_initial_registers_sample): New hook.
* libebl/eblinitreg_sample.c: New file, implements ebl interface to
  set_initial_registers_sample backend hook.
* libebl/libebl.h (ebl_set_initial_registers_sample): New function.
  (ebl_perf_frame_regs_mask): New function.
* libebl/libeblP.h (struct ebl): Add perf_frame_regs_mask field
  giving the preferred register mask.