[PATCH v2] Support for SHF_GNU_RETAIN ELF Section Flag

[Fangrui Song]

On 2020-09-28, Jozef Lawrynowicz wrote:
>The attached patch is version 2 of the SHF_GNU_RETAIN patch that was
>previously discussed here:
>https://sourceware.org/pipermail/binutils/2020-September/113406.html
>
>The following changes have been made:
>- Removed the .retain directive
>- The assembler will create different input sections for sections with
>  the same name but SHF_GNU_RETAIN set/unset (thanks to H.J. for that).
>  This means the linker will be able to do a better job garbage
>  collecting input sections, as the "retain" attribute applied to a
>  symbol declaration in the source code will not cause other parts of
>  the program that are not required, but are in the same section, to be
>  unnecessarily retained.
>- Added GNU OSABI handling (also thanks to H.J.).

My point from https://sourceware.org/pipermail/binutils/2020-September/113466.html stands.
Section flags are a bit cumbersome. If the following 

   // a.h
   __attribute__((section("sec")))
   inline void bar() { ... }
   // a.c
   #include "a.h"
   
   __attribute__((section("sec"), retain))
   void foo() {
   }

compiles to

   .section sec,"a",@progbits
   ...
   .section sec,"aR",@progbits
   ...

You will get a gas error for changing section flags
(https://sourceware.org/pipermail/binutils/2020-February/109945.html)

.reloc is really convenience in this case. You can add as many .reloc
directives as you like, each contributing one R_*_NONE to the object
file but zero cost to the linked image.