[RISCV] Drop single letter b extension support

[kito-cheng]

Summary

It splited into several zb* extensions, and b is dropped after
0.93, so it time to retired that as other non-ratified zb* extensions.

Currntly clang can accept that with warning:

$ clang -target riscv64-elf ~/hello.c -S -march=rv64gcb
'+b' is not a recognized feature for this target (ignoring feature)
'+b' is not a recognized feature for this target (ignoring feature)
'+b' is not a recognized feature for this target (ignoring feature)

[asb]

LGTM, thanks.

clang/test/Driver/riscv-arch.c

 // RV32-STD-INVAL: error: invalid arch name 'rv32imw',
 // RV32-STD-INVAL: invalid standard user-level extension 'w'
 
 // RUN: %clang --target=riscv32-unknown-elf -march=rv32imqc -### %s \
 // RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32-STD %s
 // RV32-STD: error: invalid arch name 'rv32imqc',
 // RV32-STD: unsupported standard user-level extension 'q'
 
+// RUN: %clang --target=riscv32-unknown-elf -march=rv32ib -### %s \
+// RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32-B %s
+// RV32-B: error: invalid arch name 'rv32ib',
+// RV32-B: unsupported standard user-level extension 'b'
+
 // RUN: %clang --target=riscv32-unknown-elf -march=rv32xabc -### %s \
 // RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32X %s
 // RV32X: error: invalid arch name 'rv32xabc',
 // RV32X: first letter should be 'e', 'i' or 'g'
 
 // RUN: %clang --target=riscv32-unknown-elf -march=rv32sxabc -### %s \
 // RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32SX %s
 // RV32SX: error: invalid arch name 'rv32sxabc',

llvm/lib/Support/RISCVISAInfo.cpp

if (std::next(I) != E)
       Next = std::string(std::next(I), E);
     if (auto E = getExtensionVersion(std::string(1, C), Next, Major, Minor,
                                      ConsumeLength, EnableExperimentalExtension,
                                      ExperimentalExtensionVersionCheck))
       return std::move(E);
 
     // The order is OK, then push it into features.
     // TODO: Use version number when setting target features
-    // Currently LLVM supports only "mafdcbv".
-    StringRef SupportedStandardExtension = "mafdcbv";
+    // Currently LLVM supports only "mafdcv".
+    StringRef SupportedStandardExtension = "mafdcv";
     if (!SupportedStandardExtension.contains(C))
       return createStringError(errc::invalid_argument,
                                "unsupported standard user-level extension '%c'",
                                C);
     ISAInfo->addExtension(std::string(1, C), Major, Minor);
 
     // Consume full extension name and version, including any optional '_'
     // between this extension and the next