[RISCV] Store RVC and TSO ELF flags explicitly in RISCVTargetStreamer. NFCI (#83344)

Instead of caching STI in the RISCVELFTargetStreamer, store the two
flags we need from it.

My goal is to allow RISCVAsmPrinter to override these flags using IR
module metadata for LTO. So they need to be separated from the STI used
to construct the TargetStreamer.

This patch should be NFC as long as no one is changing the contents of
the STI that was used to construct the TargetStreamer between the
constructor and the use of the flags.