RISC-V: Support -march=unset

RISC-V: Support -march=unset

This patch introduces a new `-march=unset` option for RISC-V GCC that
allows users to explicitly ignore previous `-march` options and derive
the architecture string from the `-mcpu` option instead.

This feature is particularly useful for build systems and toolchain
configurations where you want to ensure the architecture is always
derived from the CPU specification rather than relying on potentially
conflicting `-march` options.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_expand_arch):
	Ignore `unset`.
	* config/riscv/riscv.h (OPTION_DEFAULT_SPECS): Handle
	`-march=unset`.
	(ARCH_UNSET_CLEANUP_SPECS): New.
	(DRIVER_SELF_SPECS): Handle -march=unset.
	* doc/invoke.texi (RISC-V Options): Update documentation for
	`-march=unset`.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-unset-1.c: New test.
	* gcc.target/riscv/arch-unset-2.c: New test.
	* gcc.target/riscv/arch-unset-3.c: New test.
	* gcc.target/riscv/arch-unset-4.c: New test.
	* gcc.target/riscv/arch-unset-5.c: New test.