Fix TfMallocTag.Tls.Find static method on Darwin.

[furby-tm]

The original reasoning for disabling this was because originally thread_local was not available in Apple's (now ancient) Clang pre-Xcode 8, however now all modern versions of Apple's Clang do have support for thread_local.

Description of Change(s)

• The static method for TfMallocTag::Tls::Find() now returns the per-thread data for TfMallocTag on all Darwin platforms, which before this change, has always returned nothing (and was silently failing).
• Remove the ARCH_HAS_THREAD_LOCAL preprocessor.
• Remove any/all conditional compilation checks for ARCH_HAS_THREAD_LOCAL.

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Note

I had noticed the following two Tf test cases are failing, but this is unrelated to this revision, as they were failing before this revision was introduced.

99% tests passed, 2 tests failed out of 791

Total Test time (real) = 1495.16 sec

The following tests FAILED:
	 15 - TfAtomicOfstreamWrapper (Failed)
	 56 - TfSafeOutputFile (Failed)

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Fixes Issue(s)

• Compiler Warnings: Non-Void Function does not return a value #3323

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[furby-tm]

(cc. @dgovil, @AlexSchwank), I was not able to locate Nicholas Blasingame's GitHub username, but feel free to ping him here to sync up, thanks!

[jesschimein]

Filed as internal issue #USD-10127

[jesschimein]

/AzurePipelines run

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[dgovil]

Another one that we talked about on slack but I forgot to respond here. This looks like a good change to me. Thanks for solving that.

[jesschimein]

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