[MC] Rename TLSCALL as it's not ARM specific.

`MCSymbolRefExpr` variant kind for TLSCALL is prefixed with 
_ARM_ since this is how it was originally implemented.
The X86_64 version is exactly the same so there's no reason
to create a new variant, we can just rename the existing
one to be machine-independent.
This generalization is the first step to implement support
for GNU2 TLS dialect in MC.

Differential Revision:  http://reviews.llvm.org/D18160

llvm-svn: 263515