Fix riscv-linux native gdb build failure.

Fix riscv-linux native gdb build failure.

The linux kernel uses NT_PRFPREG.  Glibc before BZ 14890 defines NT_FPREGSET.
After it defines both.  Avoid glibc version dependency by using the gdb header
file instead of the glibc header file, and the macro name that gdb defines
which is NT_FPREGSET.

	gdb/
	* riscv-linux-nat.c: Include elf/common.h instead of elf.h.
	(riscv_linux_nat_target::fetch_registers): Use NT_FPREGSET instead
	of NT_PRFPREG.
	(riscv_linux_nat_target::store_registers): Likewise.