feat: Support symbol definitions in linker script

[lapla-cogito]

With --defsym support added in #1300, we can extend this to support top-level symbol definitions in linker scripts as well.

[lapla-cogito]

Sample output from the tested binary:

（▰╹◡╹）❯  readelf -s wild/tests/build/linker-script.c-default-host.ld

Symbol table '.dynsym' contains 6 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000150     0 NOTYPE  GLOBAL DEFAULT    6 start_aaa
     2: 0000000000000150     0 NOTYPE  GLOBAL DEFAULT    6 defsym_start_aaa
     3: 0000000000000154     0 NOTYPE  GLOBAL DEFAULT    6 end_bar
     4: 0000000000000148     0 NOTYPE  GLOBAL DEFAULT    6 start_bar
     5: 0000000000001234     0 NOTYPE  GLOBAL DEFAULT  ABS defsym_addr

Symbol table '.symtab' contains 12 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS linker-script.c
     2: 0000000000000148     4 OBJECT  LOCAL  DEFAULT    6 foo1
     3: 000000000000014c     4 OBJECT  LOCAL  DEFAULT    6 baz1
     4: 0000000000000150     4 OBJECT  LOCAL  DEFAULT    6 aaa1
     5: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS
     6: 0000000000000158     0 OBJECT  LOCAL  DEFAULT    7 _DYNAMIC
     7: 0000000000000148     0 NOTYPE  GLOBAL DEFAULT    6 start_bar
     8: 0000000000001234     0 NOTYPE  GLOBAL DEFAULT  ABS defsym_addr
     9: 0000000000000150     0 NOTYPE  GLOBAL DEFAULT    6 start_aaa
    10: 0000000000000150     0 NOTYPE  GLOBAL DEFAULT    6 defsym_start_aaa
    11: 0000000000000154     0 NOTYPE  GLOBAL DEFAULT    6 end_bar

（▰╹◡╹）❯  readelf -s wild/tests/build/linker-script.c-default-host.wild

Symbol table '.dynsym' contains 6 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000002838     0 NOTYPE  GLOBAL DEFAULT    7 defsym_start_aaa
     2: 000000000000283c     0 NOTYPE  GLOBAL DEFAULT    7 end_bar
     3: 0000000000001234     0 NOTYPE  GLOBAL DEFAULT  ABS defsym_addr
     4: 0000000000002838     0 NOTYPE  GLOBAL DEFAULT    7 start_aaa
     5: 0000000000002830     0 NOTYPE  GLOBAL DEFAULT    7 start_bar

Symbol table '.symtab' contains 12 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS linker-script.c
     2: 0000000000002830     4 OBJECT  LOCAL  DEFAULT    7 foo1
     3: 0000000000002834     4 OBJECT  LOCAL  DEFAULT    7 baz1
     4: 0000000000002838     4 OBJECT  LOCAL  DEFAULT    7 aaa1
     5: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT    1 __ehdr_start
     6: 0000000000002620     0 NOTYPE  GLOBAL DEFAULT    6 _DYNAMIC
     7: 0000000000002830     0 NOTYPE  GLOBAL DEFAULT    7 start_bar
     8: 0000000000002838     0 NOTYPE  GLOBAL DEFAULT    7 start_aaa
     9: 000000000000283c     0 NOTYPE  GLOBAL DEFAULT    7 end_bar
    10: 0000000000002838     0 NOTYPE  GLOBAL DEFAULT    7 defsym_start_aaa
    11: 0000000000001234     0 NOTYPE  GLOBAL DEFAULT  ABS defsym_addr

[davidlattimore]

Great to have this!

At the moment, the distinction between --symdef and the equivalent from a linker script is maintained all the way through to the write phase. Would it be possible to add the relevant symbols to the prelude earlier on, then treat them the same from then on? i.e have the code that deals with these symbols in layout and writing be the same regardless of whether the symbol came from --symdef or a linker script.

[lapla-cogito]

Thanks, top-level symbol definitions specified in linker scripts are now merged during the prelude phase. This also simplifies the processing performed by update_defsym_symbol_resolutions.