[PATCH 4.9][AArch64][testsuite] Backport r211502: PR/59843 Fix ICE on singleton vector of float on AArch64

[Alan Lawrence]

No regressions on aarch64-none-elf; new tests passing on aarch64-none-elf, 
arm-none-eabi, x86_64-unknown-linux-gnu:

NA->PASS gcc.dg/vect/vect-singleton_1.c (test for warnings, line 20)
NA->PASS gcc.dg/vect/vect-singleton_1.c (test for excess errors)

gcc/ChangeLog:

        * config/aarch64/aarch64-modes.def: Add V1DFmode.
        * config/aarch64/aarch64.c (aarch64_vector_mode_supported_p):
        Support V1DFmode.

gcc/testsuite/ChangeLog:

        * gcc.dg/vect/vect-singleton_1.c: New file.