[sanitizer] Remove st(X) from the clobber list in 32-bit x86 atomics

Summary:
When compiling with `WERROR=ON` & a recent clang, having the `st(?)` registers
in the clobber list produces a fatal error (except `st(7)` for some reason):
```
.../sanitizer_common/sanitizer_atomic_clang_x86.h:98:9: error: inline asm clobber list contains reserved registers: ST0, ST1, ST2, ST3, ST4, ST5, ST6 [-Werror,-Winline-asm]
        "movq %1, %%mm0;"  // Use mmx reg for 64-bit atomic moves
        ^
<inline asm>:1:1: note: instantiated into assembly here
        movq 8(%esp), %mm0;movq %mm0, (%esi);emms;
^
.../sanitizer_common/sanitizer_atomic_clang_x86.h:98:9: note: Reserved registers on the clobber list may not be preserved across the asm statement, and clobbering them may lead to undefined behaviour.
        "movq %1, %%mm0;"  // Use mmx reg for 64-bit atomic moves
        ^
<inline asm>:1:1: note: instantiated into assembly here
        movq 8(%esp), %mm0;movq %mm0, (%esi);emms;
^
```
As far as I can tell, they were in there due to the use of the `emms`
instruction, but removing the clobber doesn't appear to have a functional
impact. I am unsure if there is a better way to address this.

Reviewers: eugenis, vitalybuka

Reviewed By: vitalybuka

Subscribers: kubamracek, delcypher, jfb, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D50562

llvm-svn: 339575