[RISCV] Enable tools such as llvm-objdump to process objects with unrecognised base ISA versions

Tools such as llvm-objdump will currently inputs when the base ISA has
an unrecognised version. I addressed a similar issue in LLD in D144353,
introducing parseArchStringNormalized. While it would make sense to
migrate `llvm/lib/Object/ELFObjectFile.cpp` to using
`parseArchStringNormalized` as well, this patch takes a less ambitious
initial step. By tweaking the behaviour of `parseArchString` when
`IgnoreUnknown` is true (which only has one in-tree user), we use the
default supported ISA version when a base ISA with unrecognised version
is encountered.

This means that llvm-objdump and related tools will function better for
objects produced from a recent GCC. This isn't a full fix, as
IgnoreUnknown means that an imafd object with attributes specifying
newer A/F/D versions will have those extensions ignored.

Differential Revision: https://reviews.llvm.org/D146070

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