RISC-V: Add dg-remove-option for z* extensions

RISC-V: Add dg-remove-option for z* extensions

This introduces testsuite support infra for removing extensions.
Since z* extensions don't have ordering requirements the logic for
adding/removing those extensions has also been consolidated.

This fixes RVWMO compile testcases failing on Ztso targets by removing
the extension from the -march string.

gcc/ChangeLog:

	* doc/sourcebuild.texi (dg-remove-option): Add documentation.
	(dg-add-option): Add documentation for riscv_{a,zaamo,zalrsc,ztso}

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/amo/amo-table-a-6-amo-add-1.c: Add dg-remove-options
	for ztso.
	* gcc.target/riscv/amo/amo-table-a-6-amo-add-2.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-amo-add-3.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-amo-add-4.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-amo-add-5.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-compare-exchange-1.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-compare-exchange-2.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-compare-exchange-3.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-compare-exchange-4.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-compare-exchange-5.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-compare-exchange-6.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-compare-exchange-7.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-fence-1.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-fence-2.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-fence-3.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-fence-4.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-fence-5.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-load-1.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-load-2.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-load-3.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-store-1.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-store-2.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-store-compat-3.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-subword-amo-add-1.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-subword-amo-add-2.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-subword-amo-add-3.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-subword-amo-add-4.c: Ditto.
	* gcc.target/riscv/amo/amo-table-a-6-subword-amo-add-5.c: Ditto.
	* gcc.target/riscv/amo/amo-zalrsc-amo-add-1.c: Replace manually
	specified -march string with dg-add/remove-options directives.
	* gcc.target/riscv/amo/amo-zalrsc-amo-add-2.c: Ditto.
	* gcc.target/riscv/amo/amo-zalrsc-amo-add-3.c: Ditto.
	* gcc.target/riscv/amo/amo-zalrsc-amo-add-4.c: Ditto.
	* gcc.target/riscv/amo/amo-zalrsc-amo-add-5.c: Ditto.
	* lib/target-supports-dg.exp: Add dg-remove-options.
	* lib/target-supports.exp: Add dg-remove-options and consolidate z*
	extension add/remove-option code.

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