Handle .relr.dyn section

[Coekjan]

Fixes #1623

Reference: https://sourceware.org/git/?p=glibc.git;a=blob;f=elf/dynamic-link.h;h=83d834ecaf609082506d161b9744a2b476469b5e;hb=HEAD#l153

[Coekjan]

Because RELR is a new feature introduced since glibc 2.36, current ubuntu-20.04 runners may not support this feature, resulting in these compilation errors. I think we could have 2 options:

1. do something to the runner (e.g. upgrade it), and let it support RELR:
   • benefit: users who runs new programs (using RELR) will be happy.
   • drawback: to let the runner support RELR may need some engineering efforts on CI.
2. do something to the code (e.g. conditionally compile the part about RELR):
   • benefit: no need to adapt CI.
   • drawback: users' x64 apps (using RELR) can not run on these prebuilt box64 binaries.

[rajdakin]

3. You can also conditionally copy/paste the new definitions in a header (something like:

#ifndef DT_RELR
typedef struct Elf64_Relr {
    (omitted)
} Elf64_Relr;
#define DT_RELR (omitted)
#define DT_RELRSZ (omitted)
#define DT_RELRENT (omitted)
...
#endif

somewhere in the elfloader.h file)

• benefit: users who run newer programs (which use relr) will be happy
• also benefit: no need to adapt the CI

[Coekjan]

You can also conditionally copy/paste the new definitions in a header

I think this is good for now, and in the future if glibc 2.36 is widely used, we can remove these copied-pasted definitions.