Teach Sstc to respect xenvcfg.STCE

When menvcfg.STCE=0, mip.STIP reverts to its defined behavior as if
unsupporting Sstc extension. When henvcfg.STCE=0, mip.VSTIP reverts
to its defined behavior as if unsupporting Sstc extension. [https://github.com/riscv/riscv-time-compare/issues/5]

The previous Sstc implementation does not respect the xenvcfg.STCE.
In other words, the Sstc may assert mip.STIP (mip.VSTIP) when
menvcfg.STCE=0 (henvcfg.STCE=0), which is a misbehaving.