[builtins] Deduplicate __eqsf2 and __gtsf2 via macro

The only difference between __eqsf2 and __gtsf2 is whether they return
1 or -1 on NaN. Rather than duplicating all the code, use a macro to
define the function twice and use an argument to decide whether to
negate the return value.

Differential Revision: https://reviews.llvm.org/D61919

llvm-svn: 361207