Fix PR 106690: enable effective_target_bswap for RISCV targets with ZBB enabled by...

Fix PR 106690: enable effective_target_bswap for RISCV targets with ZBB enabled by default

While looking for testcases to quickly test, I Noticed that
check_effective_target_bswap was not enabled for riscv when
ZBB is enabled. This patch checks if ZBB is enabled when
targeting RISCV* for bswap.

OK? Ran the testsuite for riscv32-linux-gnu both with and without ZBB enabled.

PR testsuite/106690
gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_effective_target_bswap):
	Return true if riscv and ZBB ISA extension is enabled.