gas: Print DWARF call frame insn name in SFrame warning message

gas: Print DWARF call frame insn name in SFrame warning message

SFrame generation prints the DWARF call frame instruction opcode in
hexadecimal. Leverage get_DW_CFA_name to additionally print the
DWARF call frame instruction name in human readable form, while also
respecting fake CFI types. Use "(unknown)", if the DWARF call frame
instruction name is not known.

This changes the following assembler SFrame generation warning message
as follows:

Old:
Warning: skipping SFrame FDE due to DWARF CFI op 0x<hexnum>

New:
Warning: skipping SFrame FDE due to DWARF CFI op <name> (0x<hexnum>)

gas/
	* gen-sframe.c (sframe_get_cfi_name): New function to get the
	DWARF call frame instruction name for a DWARF call frame
	instruction opcode.
	(sframe_do_cfi_insn): Use sframe_get_cfi_name to print the
	DWARF call frame instruction name for the DWARF call frame
	instruction opcode in the warning message.

gas/testsuite/
	* gas/cfi-sframe/common-empty-1.d: Update expected SFrame
	warning message text for DWARF call frame insn name.
	* gas/cfi-sframe/common-empty-2.d: Likewise.

Reviewed-by: Andreas Krebbel <krebbel@linux.ibm.com>
Reviewed-by: Indu Bhagat <indu.bhagat@oracle.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>