gas: sframe: reset fp_reg to SFRAME_FRE_REG_INVALID

It makes sense to keep all internal state related to registers
(cfa_base_reg, fp_reg) reset to SFRAME_FRE_REG_INVALID.

While at it, also make sure fp_reg state is copied over when intializing
an SFrame FRE.

gas/
	* gen-sframe.c (sframe_row_entry_new): Reset to
	SFRAME_FRE_REG_INVALID.
	(sframe_row_entry_initialize): Copy over fp_reg too.
	(sframe_xlate_do_offset): Adjust accordingly.