Add DW_TAG_GNU_call_site* for better tail-call debug descriptions.

Add DW_TAG_GNU_call_site* for better tail-call debug descriptions.

This comes up for functions which have tail-calls optimized.
Describe location list for DW_AT_GNU_call_site_* to match readelf.
Add test elf file to demonstrate that we match readelf for DW_AT_GNU_call_site_*. C source is:
  extern int f2(int a, char b);
  extern char var;
  int f1(void) { return f2(666, var); }