[clang] [clang][RISCV] Enable RVV with function attribute __attribute__((target("arch=+v"))) (PR #83674)

[Brandon Wu via cfe-commits]

4vtomat wrote:

> FYI, the example code you shown doesn't compile anymore: https://godbolt.org/z/ooTWEGejf
> 
> This feature is quite important, without it we can't compile in RVV by default in a lot of libraries, e.g. simdutf, flac, ...

I guess it should be `__attribute__((target("arch=+zve32x")))` rather then `__attribute__((target("+zve32x")))`, sorry for misleading in the description, let me fix it.
Btw, you can check the test case in this PR to see the correct usage.

https://github.com/llvm/llvm-project/pull/83674