misc: Implement two-pass linking to separate rodata sections.

This patch changes the build to perform two-pass linking to separate rodata
with relocations (copied to RAM) from rodata with no relocations (pure data)
that can be kept in flash by llext.

For dynamic linking mode:
- First pass: Link with build-dynamic.ld to create temp ELF (unchanged).
- Analyze the elf and extract, sort and write out the sections:
    - .rodata: sections WITH relocations → copied to RAM (LLEXT_MEM_RODATA)
    - .rodata_noreloc: sections WITHOUT relocations → kept in flash (LLEXT_MEM_RODATA_NO_RELOC)
- Second pass: Link with generated rodata_split.ld + build-dynamic.ld.

For static linking mode:
- Generates empty rodata_split.ld (no section separation needed)

Signed-off-by: iabdalkader <i.abdalkader@gmail.com>

Author: iabdalkader

platform.txt

@@ -17,7 +17,7 @@ compiler.warning_flags.all=-Wall -Wextra
 
 compiler.path={build.compiler_path}
 compiler.c.cmd={build.crossprefix}gcc
-compiler.c.flags=-g -c {compiler.warning_flags} {compiler.zephyr.macros} "@{compiler.zephyr.cflags_file}" -MMD -mcpu={build.mcu} {build.float-abi} {build.fpu}
+compiler.c.flags=-g -c {compiler.warning_flags} {compiler.zephyr.macros} "@{compiler.zephyr.cflags_file}" -fdata-sections -ffunction-sections -MMD -mcpu={build.mcu} {build.float-abi} {build.fpu}
 compiler.c.elf.cmd={build.crossprefix}g++
 compiler.c.elf.flags=-Wl,--gc-sections -mcpu={build.mcu} {build.float-abi} {build.fpu} -std=gnu++17
 compiler.S.cmd={build.crossprefix}g++
@@ -58,7 +58,8 @@ build.link_command="{compiler.path}{compiler.c.elf.cmd}" "-L{build.path}" "-L{bu
 build.check_command-dynamic={build.link_command} {build.link_args.check-dynamic} -o "{build.path}/{build.project_name}_check.tmp"
 build.check_command-static=/bin/true
 build.check_command-static.windows=cmd /C cd .
-build.combine_command={build.link_command} {build.link_args.build-{build.link_mode}} {build.link_args.build-common}
+build.combine_command={build.link_command} {build.link_args.build-{build.link_mode}} "-Wl,-Map,{build.path}/{build.project_name}_temp.map" -o "{build.path}/{build.project_name}_temp.elf"
+build.combine_command_final={build.link_command} "-T{build.path}/rodata_split.ld" {build.link_args.build-{build.link_mode}} {build.link_args.build-common}
 
 # link_args.* are included by any link_command depending on the link_mode
 build.link_args.dynamic=-e main
@@ -116,9 +117,11 @@ recipe.S.o.pattern="{compiler.path}{compiler.S.cmd}" {compiler.S.flags} -DARDUIN
 ## Create archives
 recipe.ar.pattern="{compiler.path}{compiler.ar.cmd}" {compiler.ar.flags} {compiler.ar.extra_flags} "{archive_file_path}" "{object_file}"
 
-## Combine gc-sections, archives, and objects
+## Combine gc-sections, archives, and objects - Two-pass linking
 recipe.c.combine.1.pattern={build.check_command-{build.link_mode}}
 recipe.c.combine.2.pattern={build.combine_command}
+recipe.c.combine.3.pattern="{runtime.platform.path}/extra/gen_rodata_ld" "{build.path}/{build.project_name}_temp.elf" "{build.path}/rodata_split.ld" "{build.link_mode}"
+recipe.c.combine.4.pattern={build.combine_command_final}
 recipe.c.combine.pattern={build.combine_command}
 recipe.hooks.linking.postlink.1.pattern="{compiler.path}{build.crossprefix}strip" --strip-debug "{build.path}/{build.project_name}_debug.elf" -o "{build.path}/{build.project_name}.elf"
 

variants/_ldscripts/build-dynamic.ld

@@ -42,12 +42,6 @@ SECTIONS {
         __static_thread_data_list_end = .;
     }
 
-    .rodata : {
-        *(.rodata)
-        *(.rodata1)
-        *(.rodata.*)
-    }
-
     .data : {
         *(.data .data.*)
     }
@@ -82,10 +76,6 @@ SECTIONS {
         KEEP(*(.shstrtab))
     }
 
-    .rel : {
-        KEEP(*(.rel .rel.*))
-    }
-
     .got : {
         KEEP(*(.got .got.* .got.plt .got.plt*))
     }