[ELF,test] Improve tombstone value tests

Add 32-bit test for -z dead-reloc-in-nonalloc= and add tests for a
non-x86 64-bit (x86-64 is unique in discerning signed/unsigned 32-bit
absolute relocations (R_X86_64_32/R_X86_64_32S).
AArch64/PPC64/RISC-V/etc don't have the distinction). Having a test will
improve coverage for #74686