Revert "set/get_gs hack similar to set/get_fs hack"

This reverts commit e95725df03d54030065063e5ef9614eba368c5d2.

Bug: 18597541
Change-Id: I3233bd68743ae86cd98b1a97643992a0480fed7d
Fix 197259  Unsupported arch_prtctl PR_SET_GS

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r3043 | philippe | 2014-12-16 15:57:51 -0800 (Tue, 16 Dec 2014) | 21 lines

Fix 197259  Unsupported arch_prtctl PR_SET_GS option

On amd64, We handle GS similar to FS, i.e. consider it is constant.
Note that FS is not always 0 on linux. It looks rather to be constant
in all threads, and is zero in the main thread.
As values for FS and/or GS differs between platforms (linux or darwin),
FS_CONST and GS_CONST are used.

Note that we cannot easily test that the value of GS or FS is the
expected one, as the value might not be set at the begin of execution
but only set after prctl has been executed.

So, we just hope that effectively GS and FS are constant.
Some trials to set GS to other values that the expected
constant value on linux was causing a SEGV.
So, it looks like this is all effectively protected.

In summary: we were counting somewhat on the luck for FS,
we now similarly count on luch for GS

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(cherry-picked from svn://svn.valgrind.org/vex/trunk@3043)

Change-Id: Id696890f8c910187fb04e83e081512c995a0ad40
Fix 197259  Unsupported arch_prtctl PR_SET_GS

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r14815 | philippe | 2014-12-16 16:00:11 -0800 (Tue, 16 Dec 2014) | 7 lines

Fix 197259  Unsupported arch_prtctl PR_SET_GS option
(valgrind side).
In summary: we were counting somewhat on the luck for FS,
we now similarly count on luch for GS
See VEX commit log r3043 for more details.

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(cherry-picked from svn://svn.valgrind.org/valgrind/trunk@14815)

Change-Id: I29a790e0e71ed48ca0545d21efa33c6af5594aa3
Merge changes I29a790e0,Id696890f,I3233bd68

* changes:
  Fix 197259  Unsupported arch_prtctl PR_SET_GS
  Fix 197259  Unsupported arch_prtctl PR_SET_GS
  Revert "set/get_gs hack similar to set/get_fs hack"