Makefile: add multi-line target, target variable value, target separator and...

Makefile: add multi-line target, target variable value, target separator and keyword ; fix define and escaped rule

- multi-line target

```mk
target1 \
target2: ....
```

- target-specific variable value

```mk
prog: private EXTRA_CFLAGS = -L/usr/local/lib
prog: CFLAGS = -g
 #    ^ variable
 #            ^ value
```

- target separator

```mk
prog:
prog::
prog&:
prog&::
 #  ^ TargetSeparator style
```

- fix escaped rule

```mk
prog:
	echo bla \
   bla
 #^ no tab is ok
```

BUG: 470920