Subzero: Allow Makefile.standalone to build for a single target.

Makefile.standalone-help/help.txt

 Targets:
 
 check-lit - run the lit tests
 check-xtest - run the cross tests
 
- For more detailed help:
+Additional 'make' command options:
+  MINIMAL=1
+    Create a minimal build, as small and fast as possible
+  DEBUG=1
+    Compile with -O0 instead of -O2
+  NODUMP=1
+    Disable textual dump/emission support and other verbose options
+  NOASSERT=1
+    Disable assert() calls, via -DNDEBUG
+  UBSAN=1
+    Enable UBSan support, i.e. -fsanitize=undefined
+  UBSAN_TRAP=1
+    Enable UBSan support, trapping on errors
+  TSAN=1
+    Enable TSan support, i.e. -fsanitize=thread
+  ASAN=1
+    Enable ASan support, i.e. -fsanitize=address
+  MSAN=1
+    Enable MSan support, i.e. -fsanitize=memory
+  SZTARGET=<target>
+    Restrict support to a single processor target,
+    where <target> is one of {ARM32,MIPS32,X8632,X8664}
+Most of these options can be combined, e.g.
+  make -f Makefile.standalone NOASSERT=1 NODUMP=1
 
- make -f Makefile.standalone help-<target> , e.g:
- make -f Makefile.standalone help-check-lit
+For more detailed help on a 'make' target:
+
+  make -f Makefile.standalone help-<target> , e.g:
+  make -f Makefile.standalone help-check-lit