[Clang][RISCV] bfloat uses 'y' instead of 'b' (#76575)

Builtins.def says that bfloat should be represented by the 'y'
character, not the 'b' character. The 'b' character is specified to
represent boolean. The implementation currently uses 'b' correctly for
boolean and incorrectly re-uses 'b' for bfloat.

This was not caught since no builtins are emitted in
build/tools/clang/include/clang/Basic/riscv_sifive_vector_builtins.inc.
Don't know that we can test this without creating builtins that expose
this issue, although I'm not sure we really want to do that.