[ELF][PPC64]Workaround bogus Visual Studio build warning

Visual Studio has a bug where it converts the integer literal 2147483648
into an unsigned int instead of a long long (i.e. it follows C89 rules).
The bug has been reported as:
https://developercommunity.visualstudio.com/content/problem/141813/-2147483648-c4146-error.html.

Because of this bug, we were getting a signed/unsigned comparison
warning in VS2015 from the old code (the subsequent unary negation had
no effect on the type).

Reviewed by: sfertile

Differential Revision: https://reviews.llvm.org/D53821

llvm-svn: 345579