[LLD][COFF] Fix absolute & synthetic symbols in COFF symbol table

Absolute symbol should contain its absolute value, but LLD had been
writing its RVA instead. Write its VA instead.

DefinedSynthetic were being skipped before with the reasoning "Relative
symbols are unrepresentable in a COFF symbol table", which is only true
if the RVA points to outside of a section. LLD does create synthetic
symbols which points to actual data chunks (typical for symbols embedded
into the load config directory). Write these symbols to the COFF symbol
table too.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D134462