Apply a fix, extracted from the UCB RISC-V gcc sources, for the (known)
problems with compiling atomics for the riscv64 target with gcc.  Remove
the now-unneeded workarounds for the 2 files in libasan that were formerly
failing to compile with the optimizer turned on.
Remove note of hack around for riscv compiler problems with
atomics.  This seems to now be fixed.
fix A20/A31 MP