fix the manual config and flipped condition in SpinLock.h

fix the manual config and flipped condition in SpinLock.h

Summary:
i) FOLLY_HAVE_INT128_T definition is duplicated and is getting set to 1 on android. deleted that.
ii) we dont have pthread_spinlock_t and pthread_atfork on android, the former is absent apple as well
iii) a condition got flipped in D1768669. __ANDROID__ implies !FOLLY_HAVE_PTHREAD_SPINLOCK_T. fixed that

Test Plan:
- fbconfig -r folly && fbmake
- ran folly build with buck as well
- synced to fb4a and it builds

Reviewed By: seanc@fb.com

Subscribers: doug, seanc, folly-diffs@

FB internal diff: D1783579

Signature: t1:1783579:1421350138:9301303f77784e7bdb851e3f643751078457b025