[X86] Support for the mno-tls-direct-seg-refs flag

Allows to disable direct TLS segment access (%fs or %gs). GCC supports
a similar flag, it can be useful in some circumstances, e.g. when a thread
context block needs to be updated directly from user space. More info
and specific use cases: https://bugs.llvm.org/show_bug.cgi?id=16145

There is another revision for clang as well.
Related: D53102

All X86 CodeGen tests appear to pass:
```
[46/47] Running lit suite /SourceCache/llvm-trunk-8.0/test/CodeGen
Testing Time: 23.17s
  Expected Passes    : 3801
  Expected Failures  : 15
  Unsupported Tests  : 8021
```

Reviewed by: Craig Topper.

Patch by nruslan (Ruslan Nikolaev).

Differential Revision: https://reviews.llvm.org/D53103

llvm-svn: 344723

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index e977657d1cb..d396e3f1cbf 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -1450,6 +1450,10 @@ example:
 ``noredzone``
     This attribute indicates that the code generator should not use a
     red zone, even if the target-specific ABI normally permits it.
+``indirect-tls-seg-refs``
+    This attribute indicates that the code generator should not use
+    direct TLS access through segment registers, even if the
+    target-specific ABI normally permits it.
 ``noreturn``
     This function attribute indicates that the function never returns
     normally. This produces undefined behavior at runtime if the