Don't explicitly represent x0 in sigcontext etc.

Move epc into that slot instead.

Signed-off-by: Albert Ou <a_ou@eecs.berkeley.edu>
Adjust ignore pattern to match README.md file name change.

Signed-off-by: Albert Ou <a_ou@eecs.berkeley.edu>
riscv: Improve atomic memory access and support -mno-atomic.

Use CONFIG_GENERIC_ATOMIC64 if 32-bit.  Add system call to emulate
atomic compare exchange, primarily for targets lacking hardware
suport (i.e., lacking RV32A, RV64A).  Support excluding use of
atomic memory instructions (-mno-atomic).

This change is mutually dependent on the corresponding change to
riscv-gcc.

Signed-off-by: Albert Ou <a_ou@eecs.berkeley.edu>
sysriscv: Propagate original uaccess error codes