[tilegx] Market RegExpExecStub::Generate as UNREACHABLE.

  Currently we use interpreted RegExp, no JIT support.

  Mark as UNREACHABLE so that if it's triggered we could know
immediately.
[tilegx] Fix one coding style.
[tilegx] Tempararily disable GC sanity check.

  When this check enabled, it may fail, but when disabled
those related testcases could always run through successfully.

  So, temparaily disable this check, will go back on this issue later.
[tilegx] Fix FullCodeGenerator::EmitIsStringWrapperSafeForDefaultValueOf bug.

  * what generated by NumberOfOwnDescriptors is in SMI foramt, thus
    we need to De-SMI first in the following operations.
  * what stored in kHashFieldOffset field is actually 4byte.

  As of this commit, the testsuite pass/fail result is 8820/903.