Fix a preprocessor warning exposed by -Wundef.

Bug: 18589470

__LP64__ is only defined for 64-bit compilation targets, so this macro
is evaluated as an implicit 0 in the original case. This change makes
it explicit that we only care whether this is actually defined.

Change-Id: I8c11b41fc0bde9477c5efeea0782d9741d9d827d