RISC-V: Fix __riscv_compressed regression.

RISC-V: Fix __riscv_compressed regression.

This was accidentally broken by the patch adding ELF attribute support.
The __riscv_compressed predefined macro is not being defined when the
target supports the C extension.  The fix adds testcases to verify that
all of the predefined macros are correct for various option combinations.

	gcc/
	* common/config/riscv/riscv-common.c (riscv_parse_arch_string):
	Clear MASK_RVC and then set if C subset supported.

	gcc/testsuite/
	* gcc.target/riscv/predef-1.c: New.
	* gcc.target/riscv/predef-2.c: New.
	* gcc.target/riscv/predef-3.c: New.
	* gcc.target/riscv/predef-4.c: New.
	* gcc.target/riscv/predef-5.c: New.
	* gcc.target/riscv/predef-6.c: New.
	* gcc.target/riscv/predef-7.c: New.
	* gcc.target/riscv/predef-8.c: New.

From-SVN: r270026