[PATCH] RISC-V: Deduplicate #error messages in testsuite

"#error Feature macro not defined" is required to test the existence of an
extension through the preprocessor.  However, multiple occurrence of the
exact same error message will confuse the developer once an error is
encountered.

This commit replaces such error messages to
"#error Feature macro for `EXT' not defined" to make which
macro is missing.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/zvkn.c: Deduplicate #error messages.
	* gcc.target/riscv/zvkn-1.c: Ditto.
	* gcc.target/riscv/zvknc.c: Ditto.
	* gcc.target/riscv/zvknc-1.c: Ditto.
	* gcc.target/riscv/zvknc-2.c: Ditto.
	* gcc.target/riscv/zvkng.c: Ditto.
	* gcc.target/riscv/zvkng-1.c: Ditto.
	* gcc.target/riscv/zvkng-2.c: Ditto.
	* gcc.target/riscv/zvks.c: Ditto.
	* gcc.target/riscv/zvks-1.c: Ditto.
	* gcc.target/riscv/zvksc.c: Ditto.
	* gcc.target/riscv/zvksc-1.c: Ditto.
	* gcc.target/riscv/zvksc-2.c: Ditto.
	* gcc.target/riscv/zvksg.c: Ditto.
	* gcc.target/riscv/zvksg-1.c: Ditto.
	* gcc.target/riscv/zvksg-2.c: Ditto.