RISC-V: Add FP register core file support.

RISC-V: Add FP register core file support.

This adds fp reg support similar to the existing general reg support.

This fixes one gdb testsuite failure
FAIL: gdb.base/gcore.exp: corefile restored system registers
which fails without the patch because fcsr was missing.  Otherwise, no
regressions with riscv64-linux native testsuite run.

	gdb/
	* riscv-linux-tdep.c (riscv_linux_fregmap): New.
	(riscv_linux_fregset): New.
	(riscv_linux_iterate_over_regset_sections): Call cb for .reg2 section.