Allow specifying key "llvm-abiname" in target specification

Allow specifying key "llvm-abiname" in target specification

This addresses #65024, as it allows RISC-V target specification
files to set "llvm-abiname": "lp64d". In general, it is useful
for the programmer to be able to set this codegen parameter,
which other languages usually expose under a compiler argument
like "-mabi=<XYZ>".