RISC-V: Linux signal frame support.

RISC-V: Linux signal frame support.

Make riscv_isa_flen available to the linux native code, and clean up duplicate
comments.

	gdb/
	* riscv-tdep.c (riscv_isa_xlen): Refer to riscv-tdep.h comment.
	(riscv_isa_flen): Likewise.  Drop static.
	* riscv-tdep.h (riscv_isa_xlen): Move riscv-tdep.c comment to here.
	(riscv_isa_flen): Likewise.