ld/pe-dll: Don't auto-export symbols from .tls section

The .tls section of an image contains template data that will be duplicated
for each new thread to be created. Accessing thread-local data involves an
image-specific global variable, _tls_used, for calculation. Therefore, it is
impossible to export thread-local variables from a DLL.

The only symbol that exists in the .tls input section is _tls_start, but it's
a CRT symbol and is never auto-exported. It's only necessary to check for
user symbols, which are always generated in the subsection .tls$ by default,
or in subsections like .tls$$<symbol> when -fdata-sections is specified.

Reference: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80881
Signed-off-by: LIU Hao <lh_mouse@126.com>