[ELF] Fix TP offset of TLS Variant I after D62059

As Ryan Prichard pointed out, after D62059, the TP offset is incorrect.

Add x86-64-tls-le-align.s to check this.  Better formulae for both
variants should take p_vaddr%p_align into account (offset%p_align =
p_vaddr%p_align is a basic ELF requirement), but I can't find a way to
test the behavior.

llvm-svn: 361084