cmake: linker_script: fix __exidx_start

This makes c++ sample
`sample.libraries.hash_map.newlib.cxx_unordered_map.djb2`
start working on gcc w. `CMAKE_LINKER_GENERATOR=y`.

Signed-off-by: Robin Kastberg <robin.kastberg@iar.com>

cmake/linker_script/arm/linker.cmake

@@ -125,7 +125,7 @@ endif()
 zephyr_linker_section(NAME .ARM.exidx GROUP ROM_REGION)
 # Here the original linker would check for __GCC_LINKER_CMD__, need to check toolchain linker ?
 #if(__GCC_LINKER_CMD__)
-  zephyr_linker_section_configure(SECTION .ARM.exidx INPUT ".gnu.linkonce.armexidx.*")
+  zephyr_linker_section_configure(SECTION .ARM.exidx INPUT ".gnu.linkonce.armexidx.*" SYMBOLS "__exidx_start" "__exidx_end")
 #endif()