[asan] Fix incorrect SEH symbol mangling on win64.

Summary:
The ASAN unittests are failing (check-asan-dynamic) due to an incorrect symbol name:
```
LINK : error LNK2001: unresolved external symbol ___asan_seh_interceptor
```

On win64, the linker is not adding an extra underscore. This was correctly fixed in the same file for other uses.

After that patch, most of the unittests are passing, but some related to SEH needs to be fixed.
```
Failing Tests (4):
    AddressSanitizer-x86_64-windows-dynamic :: TestCases/Windows/dll_intercept_memchr.cc
    AddressSanitizer-x86_64-windows-dynamic :: TestCases/Windows/dll_intercept_memcpy_indirect.cc
    AddressSanitizer-x86_64-windows-dynamic :: TestCases/Windows/dll_seh.cc
    AddressSanitizer-x86_64-windows-dynamic :: TestCases/Windows/seh.cc

  Expected Passes    : 339
  Passes With Retry  : 3
  Expected Failures  : 16
  Unsupported Tests  : 152
  Unexpected Failures: 4
```

Reviewers: rnk, kcc, majnemer

Subscribers: majnemer, chrisha, cfe-commits

Differential Revision: https://reviews.llvm.org/D24841

llvm-svn: 282251