[AArch64] Only update assembler .arch directive when necessary
* config/aarch64/aarch64.c (aarch64_last_printed_arch_string):
New variable.
(aarch64_last_printed_tune_string): Likewise.
(aarch64_declare_function_name): Only output .arch assembler
directive if it will be different from the previously output
directive. Same for .tune comment but only if -dA is set.
(aarch64_start_file): New function.
(TARGET_ASM_FILE_START): Define.
* gcc.target/aarch64/target_attr_15.c: Scan assembly for
.arch armv8-a\n. Add -dA to dg-options.
* gcc.target/aarch64/assembler_arch_1.c: New test.
* gcc.target/aarch64/target_attr_7.c: Add -dA to dg-options.
From-SVN: r233342