atomics A.1b: real RMW emission (atomicrmw) + unit test (green)

emitAtomicRmw: LLVMBuildAtomicRMW (binop from RmwKind; signed Min/Max vs
unsigned UMin/UMax from val_ty; singleThread=0; LLVM supplies ABI alignment).
examples/1701 green (add/sub/and/or/xor/min/max return old values, results
verified). Unit test 'emit: atomic rmw (add + signed/unsigned min)' locks
'atomicrmw add' + signed 'min' vs unsigned 'umin'. Suite green (716/0).

examples/expected/1701-atomics-rmw.exit

@@ -1 +1 @@
-1
+0

examples/expected/1701-atomics-rmw.stderr

@@ -1,7 +1 @@
-error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
-error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
-error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
-error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
-error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
-error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
-error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
+

examples/expected/1701-atomics-rmw.stdout

@@ -1 +1,10 @@
-
+old add: 10
+old sub: 15
+now: 12
+old and: 240
+old or:  48
+old xor: 51
+now: 60
+old min: 20
+old max: 8
+now: 15