objtool: Fix up st_info in COMDAT group section

When __elf_create_symbol creates a local symbol, it relocates the first
global symbol upwards to make space. Subsequently, elf_update_symbol()
is called to refresh the symbol table section. However, this isn't
sufficient, as other sections might have the reference to the old
symbol index, for instance, the sh_info field of an SHT_GROUP section.

This patch updates the `sh_info` field when necessary. This field
serves as the key for the COMDAT group. An incorrect key would prevent
the linker's from deduplicating COMDAT symbols, leading to duplicate
definitions in the final link.

Signed-off-by: Rong Xu <xur@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250425200541.113015-1-xur@google.com