RISC-V: Update testsuite to use b

RISC-V: Update testsuite to use b

Update all instances of zba_zbb_zbs in the testsuite to use b instead

gcc/testsuite/ChangeLog:

	* g++.target/riscv/redundant-bitmap-1.C: Use gcb instead of
	zba_zbb_zbs
	* g++.target/riscv/redundant-bitmap-2.C: Ditto
	* g++.target/riscv/redundant-bitmap-3.C: Ditto
	* g++.target/riscv/redundant-bitmap-4.C: Ditto
	* gcc.target/riscv/shift-add-1.c: Ditto
	* gcc.target/riscv/shift-add-2.c: Ditto
	* gcc.target/riscv/synthesis-1.c: Ditto
	* gcc.target/riscv/synthesis-2.c: Ditto
	* gcc.target/riscv/synthesis-3.c: Ditto
	* gcc.target/riscv/synthesis-4.c: Ditto
	* gcc.target/riscv/synthesis-5.c: Ditto
	* gcc.target/riscv/synthesis-6.c: Ditto
	* gcc.target/riscv/synthesis-7.c: Ditto
	* gcc.target/riscv/synthesis-8.c: Ditto
	* gcc.target/riscv/zba_zbs_and-1.c: Ditto
	* gcc.target/riscv/zbs-zext-3.c: Ditto
	* lib/target-supports.exp: Add b to riscv_get_arch

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