[ELF] Guard --fix-cortex-a53-843419 against --just-syms

If --just-syms is used the mapping symbols from the ELF file will be
absolute symbols with no section. The code to process mapping symbols in
--fix-cortex-a53-843419 assumes that these symbols have a defining section
so a crash will result when --just-syms is used. The simple fix is to not
process the symbol when it doesn't have a section.

Fixes PR37971

Differential Revision: https://reviews.llvm.org/D52038

llvm-svn: 342146