RISC-V: Require a extension for testcases with atomic insns

RISC-V: Require a extension for testcases with atomic insns

Add testsuite infrastructure for the A extension and use it to require the A
extension for dg-do run and add the add extension for non-A dg-do compile.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/amo-table-a-6-amo-add-1.c: Add A extension to
	dg-options for dg-do compile.
	* gcc.target/riscv/amo-table-a-6-amo-add-2.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-amo-add-3.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-amo-add-4.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-amo-add-5.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-compare-exchange-1.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-compare-exchange-2.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-compare-exchange-3.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-compare-exchange-4.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-compare-exchange-5.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-compare-exchange-6.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-compare-exchange-7.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-subword-amo-add-1.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-subword-amo-add-2.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-subword-amo-add-3.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-subword-amo-add-4.c: Ditto.
	* gcc.target/riscv/amo-table-a-6-subword-amo-add-5.c: Ditto.
	* gcc.target/riscv/inline-atomics-2.c: Ditto.
	* gcc.target/riscv/inline-atomics-3.c: Require A extension for dg-do
	run.
	* gcc.target/riscv/inline-atomics-4.c: Ditto.
	* gcc.target/riscv/inline-atomics-5.c: Ditto.
	* gcc.target/riscv/inline-atomics-6.c: Ditto.
	* gcc.target/riscv/inline-atomics-7.c: Ditto.
	* gcc.target/riscv/inline-atomics-8.c: Ditto.
	* lib/target-supports.exp: Add testing infrastructure to require the A
	extension or add it to an existing -march.

Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>