RISC-V: Add configure check for B extention support

Binutils 2.42 and before don't recognize the b extension in the march
strings even though it supports zba_zbb_zbs. Add a configure check to
ignore the b in the march string if found.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_subset_list::to_string):
	Skip b in march string
	* config.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Add B assembler check

Signed-off-by: Edwin Lu <ewlu@rivosinc.com>