Add missing builtins to altivec.h for ABI compliance (vol. 1)

This patch corresponds to review:
http://reviews.llvm.org/D10637

This is the first round of additions of missing builtins listed in the ABI document. More to come (this builds onto what seurer already addes). This patch adds:
vector signed long long vec_abs(vector signed long long)
vector double vec_abs(vector double)
vector signed long long vec_add(vector signed long long, vector signed long long)
vector unsigned long long vec_add(vector unsigned long long, vector unsigned long long)
vector double vec_add(vector double, vector double)
vector double vec_and(vector bool long long, vector double)
vector double vec_and(vector double, vector bool long long)
vector double vec_and(vector double, vector double)
vector signed long long vec_and(vector signed long long, vector signed long long)
vector double vec_andc(vector bool long long, vector double)
vector double vec_andc(vector double, vector bool long long)
vector double vec_andc(vector double, vector double)
vector signed long long vec_andc(vector signed long long, vector signed long long)
vector double vec_ceil(vector double)
vector bool long long vec_cmpeq(vector double, vector double)
vector bool long long vec_cmpge(vector double, vector double)
vector bool long long vec_cmpge(vector signed long long, vector signed long long)
vector bool long long vec_cmpge(vector unsigned long long, vector unsigned long long)
vector bool long long vec_cmpgt(vector double, vector double)
vector bool long long vec_cmple(vector double, vector double)
vector bool long long vec_cmple(vector signed long long, vector signed long long)
vector bool long long vec_cmple(vector unsigned long long, vector unsigned long long)
vector bool long long vec_cmplt(vector double, vector double)
vector bool long long vec_cmplt(vector signed long long, vector signed long long)
vector bool long long vec_cmplt(vector unsigned long long, vector unsigned long long)


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240821 91177308-0d34-0410-b5e6-96231b3b80d8