Use HOST_CFLAGS more consistently

HOST_CFLAGS which may contain MacOSX SDK and flags is now
used by all build-host*.sh

Also fixed:
1. Compile GCC with -O2 !!
2. Bump up SDK to 10.6 (the min version of SDK/NDK for a while)
3. run llvm and polly test but don't fail when ont all test pass
4. Remove link to polly
5. No need to show "Using C compiler" "Using C++ compiler" for --mingw
   which uses wrapper

Change-Id: I25298529d40b6a09530aa35aa2ec2cb3223857e3
5 files changed