linux-user/elfload: Tidy fill_note_info and struct elf_note_info

linux-user/elfload: Tidy fill_note_info and struct elf_note_info

In fill_note_info, there were unnecessary checks for
success of g_new/g_malloc.  But these structures do not
need to be dyamically allocated at all, and can in fact
be statically allocated within the parent structure.

This removes all error paths from fill_note_info, so
change the return type to void.

Change type of signr to match both caller (elf_core_dump)
and callee (fill_prstatus), which both use int for signr.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>