RISC-V: Define __riscv_cmodel_medany for PIC mode.

RISC-V: Define __riscv_cmodel_medany for PIC mode.

 - According the conclusion in RISC-V C API document, we decide to deprecat
   the __riscv_cmodel_pic marco

 - __riscv_cmodel_pic is deprecated and will removed in next GCC
   release.

[1] https://github.com/riscv/riscv-c-api-doc/pull/11

gcc/ChangeLog:

	* config/riscv/riscv-c.c (riscv_cpu_cpp_builtins): Define
	__riscv_cmodel_medany when PIC mode.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/predef-3.c: Update testcase.
	* gcc.target/riscv/predef-6.c: Ditto.