Clear vtop.sym if saving on stack
Clear vtop.sym if saving on stack

normally the sym slot is meaningful only with VT_SYM.  But we also
use it when mentioning a decl for inline asms with register vars,
conditional on being a VT_LOCAL entry.  So when generating VT_LOCAL we
need to reset .sym as it might contain stale entries from the cmp_op
fields.

mirror/tinycc,mirror/tinycc,mirror/tinycc,mingodad/tinycc,mirror/tinycc,mingodad/tinycc,mingodad/tinycc,mingodad/tinycc