[RISCV][NFC] Reimplementation of target attribute override mechanism (#106680)
This patch aims to replace the target attribute override mechanism based
on `__RISCV_TargetAttrNeedOverride` with the insertion of several
negative target features
When the target attribute uses the full architecture string
("arch=rv64gc") or specifies the CPU ("cpu=rocket-rv64") as the version,
it will override the module-level target feature. Currently, this
mechanism is implemented by inserting `__RISCV_TargetAttrNeedOverride`
as a dummy target feature immediately before the target attribute's
feature.
```
module target features + __RISCV_TargetAttrNeedOverride + target attribute's feature
```
The RISCVTargetInfo::initFeatureMap function will remove the "module
target features" and use only the "target attribute's features".
This patch changes the process as follows:
```
module target features + negative target feature for all supported extension + target attribute's feature
```
The `module target features` will be disable by `negative target feature
for all supported extension` in `TargetInfo::initFeatureMap`
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