RISC-V: Attribute parser: Use alloca() instead of new + std::unique_ptr

RISC-V: Attribute parser: Use alloca() instead of new + std::unique_ptr

Allocating an object on the heap with new, wrapping it in a
std::unique_ptr and finally getting the buffer via buf.get()
is a correct way to allocate a buffer that is automatically
freed on return.  However, a simple invocation of alloca()
does the same with less overhead.

gcc/ChangeLog:

	* config/riscv/riscv-target-attr.cc (riscv_target_attr_parser::parse_arch):
	Replace new + std::unique_ptr by alloca().
	(riscv_process_one_target_attr): Likewise.
	(riscv_process_target_attr): Likewise.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>