[compiler-rt][RISCV] Implement __riscv_feature_bits/__riscv_vendor_feature_bits/__init_riscv_features_bit

Base on riscv-non-isa/riscv-c-api-doc#74, this patch defines the __riscv_feature_bits and __riscv_vendor_feature_bits structures to store the enabled feature bits at runtime.

It also introduces the __init_riscv_features_bit function to update these structures based on the platform query mechanism.

Additionally, the groupid/bitmask definitions from riscv-non-isa/riscv-c-api-doc#74 are declared and used to update the __riscv_feature_bits and __riscv_vendor_feature_bits structures.

Author: BeMg

compiler-rt/lib/builtins/CMakeLists.txt

@@ -716,6 +716,7 @@ endif()
 set(powerpc64le_SOURCES ${powerpc64_SOURCES})
 
 set(riscv_SOURCES
+  riscv/ifunc_select.c
   riscv/fp_mode.c
   riscv/save.S
   riscv/restore.S