[X86] Slightly refactor default features for AMD bdver cpus (NFC). Also add missing checks to test for target features.

This patch simplifies how default target features are set for AMD bdver2
and bdver1. In particular, method 'getDefaultFeatures' now implements a
fallthrough from case 'CK_BDVER2' to case 'CK_BDVER1'.
That is because 'bdver2' has the same features available in bdver1 plus
BMI, FMA, F16C and TBM.

This patch also adds missing checks for predefined macros in test
predefined-arch-macros.c. In the case of BTVER2, the test now also checks
for F16C, BMI and PCLMUL. In the case of BDVER3 and BDVER4, the test now
also checks for the presence of FSGSBASE.

Differential Revision: http://reviews.llvm.org/D6134