gdb/riscv: make riscv target description names global

gdb/riscv: make riscv target description names global

A later commit will need the names of the RISC-V target description
features in files other than riscv-tdep.c.  This commit just makes the
names global strings that can be accessed from other riscv-*.c files.

There should be no user visible changes after this commit.

gdb/ChangeLog:

        * riscv-tdep.c (riscv_feature_name_csr): Define.
        (riscv_feature_name_cpu): Define.
        (riscv_feature_name_fpu): Define.
        (riscv_feature_name_virtual): Define.
        (riscv_xreg_feature): Use riscv_feature_name_cpu.
        (riscv_freg_feature): Use riscv_feature_name_fpu.
        (riscv_virtual_feature): Use riscv_feature_name_virtual.
        (riscv_csr_feature): Use riscv_feature_name_csr.
        * riscv-tdep.h (riscv_feature_name_csr): Declare.