Add syscall thread_read_state

[GCYYfun]

Is there any special reason for not adding this syscall, as I see the underlying implementation is already complete.
If it's just missing, then I'll add it!

[wangrunji0408]

Working on #167

[coveralls]

Pull Request Test Coverage Report for Build 234452318

• 0 of 10 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 49.007%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
zircon-syscall/src/task.rs	0	10	0.0%

Totals
Change from base Build 230236257:	-0.02%
Covered Lines:	7401
Relevant Lines:	15102

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 234452318

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 10 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 49.007%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
zircon-syscall/src/task.rs	0	10	0.0%

Totals
Change from base Build 230236257:	-0.02%
Covered Lines:	7401
Relevant Lines:	15102

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 234452318

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 10 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 49.007%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
zircon-syscall/src/task.rs	0	10	0.0%

Totals
Change from base Build 230236257:	-0.02%
Covered Lines:	7401
Relevant Lines:	15102

---

💛 - Coveralls