RISC-V: Don't make Ztso imply A

I can't actually find anything in the ISA manual that makes Ztso imply
A.  In theory the memory ordering is just a different thing that the set
of availiable instructions (ie, Ztso without A would still imply TSO for
loads and stores).  It also seems like a configuration that could be
sane to build: without A it's all but impossible to write any meaningful
multi-core code, and TSO is really cheap for a single core.

That said, I think it's kind of reasonable to provide A to users asking
for Ztso.  So maybe even if this was a mistake it's the right thing to
do?

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_implied_info):
	Remove {"ztso", "a"}.