[llvm] [RISCV] Use 'riscv-isa' module flag to set ELF flags and attributes. (PR #85155)

[Paul Kirth via llvm-commits]

ilovepi wrote:

Doesnt' this fix the issue w/ the Float ABI in LTO, where some LTO'd targets wouldn't get the ABI propagated? Also, if we land this in the release branch, Rust will pick it up, which is probably a win for anyone LTOing Rust code.

https://github.com/llvm/llvm-project/pull/85155