[MC] Refactor emitELFSize to make usage more consistent. NFC.

[MC] Refactor emitELFSize to make usage more consistent. NFC.

Move the cast<MCSymbolELF> inside emitELFSize, so that:
 - it's done in one place instead of at each call
 - it's more consistent with similar functions like EmitCOFFSafeSEH
 - ambiguity between cast<> and dyn_cast<> is avoided (which also
   eliminates an unnecessary dyn_cast call)

This also makes it easier to experiment with using ".size" directives on
non-ELF targets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288437 91177308-0d34-0410-b5e6-96231b3b80d8