configure.ac: explicitly check for asm/perf_regs.h presence

configure.ac: explicitly check for asm/perf_regs.h presence

Not all Linux arches include <asm/perf_regs.h>. Checking for just
architecture (__x86__/__i386__) has turned out to be brittle, so
now we add configury.

* configure.ac: Introduce HAVE_PERF_REGS_H.
* backends/i386_initreg_sample.c: Guard asm/perf_regs.h include.
* backends/x86_64_initreg_sample.c: Ditto.
* backends/libebl_PERF_FLAGS.h: Ditto.
* src/stacktrace.c: Clarify that eu-stacktrace configury currently
  conditions the asm/perf_regs.h include on _ASM_X86_PERF_REGS_H.