Drop __riscv_vendor_feature_bits

As discussed in #96, current interface is insufficient to support some
cases, like a vendor buying a CPU IP from the upstream vendor but using
their own mvendorid and custom features from the upstream vendor. In
this case, we might need to add these extensions for each downstream
vendor many times. Thus, making __riscv_vendor_feature_bits guarded by
mvendorid is not a good idea. So, drop __riscv_vendor_feature_bits for
now, and we should have time to discuss a better solution.

Author: cyyself

src/c-api.adoc

@@ -839,7 +839,7 @@ This feature is activated by the `target_version/target_clones` function attribu
 
 === Extension Bitmask
 
-The Extension Bitmask is used to probe whether features are enabled during runtime. This is achieved through three bitmask structures: `__riscv_feature_bits` for standard extensions,  `__riscv_vendor_feature_bits` for vendor-specific extensions and `__riscv_cpu_model` for the CPU model. Additionally, `__init_riscv_feature_bits` is used to update the contents of these structures according to the system configuration.
+The Extension Bitmask is used to probe whether features are enabled during runtime. This is achieved through three bitmask structures: `__riscv_feature_bits` for standard extensions and `__riscv_cpu_model` for the CPU model. Additionally, `__init_riscv_feature_bits` is used to update the contents of these structures according to the system configuration.
 
 The bitmask structures use the following definitions:
 
@@ -849,11 +849,6 @@ struct {
     unsigned long long features[];
 } __riscv_feature_bits;
 
-struct {
-    unsigned length;
-    unsigned long long features[];
-} __riscv_vendor_feature_bits;
-
 struct {
   unsigned mvendorid;
   unsigned long long marchid;
@@ -873,7 +868,7 @@ To initiate these structures based on the system's extension status, the followi
 void __init_riscv_feature_bits(void *);
 ```
 
-The `__init_riscv_feature_bits` function updates `length`, `mvendorid`, `marchid`, `mimpid` and the `features` in `__riscv_feature_bits` and `__riscv_vendor_feature_bits` according to the enabled extensions in the system.
+The `__init_riscv_feature_bits` function updates `length`, `mvendorid`, `marchid`, `mimpid` and the `features` in `__riscv_feature_bits` according to the enabled extensions in the system.
 
 The `__init_riscv_feature_bits` function accepts an argument of type `void *`. This argument allows the platform to provide pre-computed data and access it without additional effort. For example, Linux could pass the vDSO object to avoid an extra system call.