aarch64: Fix SMSTART/SMSTOP save/restore for BE

aarch64: Fix SMSTART/SMSTOP save/restore for BE

VNx16QI (the SVE register byte mode) is the only SVE mode for which
LD1 and LDR result in the same register layout for big-endian.  It is
therefore the only mode for which we allow LDR and STR to be used for
big-endian SVE moves.

The SME support sometimes needs to use LDR and STR to save and restore
Z register contents around an SMSTART/SMSTOP SM.  It therefore needs
to use VNx16QI regardless of the type of value that is stored in the
Z registers.

gcc/
	PR target/112930
	* config/aarch64/aarch64.cc (aarch64_sme_mode_switch_regs::add_reg):
	Force specific SVE modes for single registers as well as structures.