[DWARF5] Replace emission of strp with stx forms in debug_macro section

DW_MACRO_define_strx forms are supported now in llvm-dwarfdump and these
forms can be used in both debug_macro[.dwo] sections. An added advantage
for using strx forms over strp forms is that it uses indices
approach instead of a relocation to debug_str section.

This patch unify the emission for debug_macro section.

Reviewed by: dblaikie, ikudrin

Differential Revision: https://reviews.llvm.org/D78865