[rs6000] Support more prototypes for vec_ld and vec_st.

[rs6000] Support more prototypes for vec_ld and vec_st.

2019-01-17  Kewen Lin  <linkw@gcc.gnu.org>

gcc/ChangeLog
    * doc/extend.texi: Add four new prototypes for vec_ld and seven new
    prototypes for vec_st.
    * config/rs6000/rs6000-c.c (altivec_overloaded_builtins): Add entries
    for scalar address type variants of altivec_vec_ld/altivec_vec_st,
    mainly on signed/unsigned long long and double.

gcc/testsuite/ChangeLog
    * gcc.target/powerpc/altivec_vld_vst_addr.c: New test.

From-SVN: r268007