[RISCV][FMV] Support target_clones (#85786)

This patch enable the function multiversion(FMV) and `target_clones`
attribute for RISC-V target.

The proposal of `target_clones` syntax can be found at the
https://github.com/riscv-non-isa/riscv-c-api-doc/pull/48 (which has
landed), as modified by the proposed
https://github.com/riscv-non-isa/riscv-c-api-doc/pull/85 (which adds the
priority syntax).

It supports the `target_clones` function attribute and function
multiversioning feature for RISC-V target. It will generate the ifunc
resolver function for the function that declared with target_clones
attribute.

The resolver function will check the version support by runtime object
`__riscv_feature_bits`.

For example:

```
__attribute__((target_clones("default", "arch=+ver1", "arch=+ver2"))) int bar() {
    return 1;
}
```

the corresponding resolver will be like:

```
bar.resolver() {
    __init_riscv_feature_bits();
    // Check arch=+ver1
    if ((__riscv_feature_bits.features[0] & BITMASK_OF_VERSION1) == BITMASK_OF_VERSION1) {
        return bar.arch=+ver1;
    } else {
        // Check arch=+ver2
        if ((__riscv_feature_bits.features[0] & BITMASK_OF_VERSION2) == BITMASK_OF_VERSION2) {
            return bar.arch=+ver2;
        } else {
            // Default
            return bar.default;
        }
    }
}
```

1 files changed, 4 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 50fa486..ba2d658 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -4283,7 +4283,10 @@ void CodeGenModule::emitMultiVersionFunctions() {
               Feats.clear();
               if (getTarget().getTriple().isAArch64())
                 TC->getFeatures(Feats, I);
-              else {
+              else if (getTarget().getTriple().isRISCV()) {
+                StringRef Version = TC->getFeatureStr(I);
+                Feats.push_back(Version);
+              } else {
                 StringRef Version = TC->getFeatureStr(I);
                 if (Version.starts_with("arch="))
                   Architecture = Version.drop_front(sizeof("arch=") - 1);