[GCC][BUG][ARM] Fix ICE due to BFmode libfunc call (PR93300)

[GCC][BUG][ARM] Fix ICE due to BFmode libfunc call (PR93300)

    This was sent and approved on gcc-patches as "[GCC][BUG][Aarch64][ARM]
    (PR93300) Fix ICE due to BFmode placement in GET_MODES_WIDER chain".

    The observed error came about because BFmode was placed between HFmode
    and SFmode in the GET_MODES_WIDER chain, resulting in convert_mode_scalar
    attempting to gen a libfunc for a HFmode -> BFmode conversion.

    This patch registers NULL for all libfuncs in BFmode, which stops the
    middle-end from attempting to generate them.

    gcc/ChangeLog:
    2020-02-06  Stam Markianos-Wright  <stam.markianos-wright@arm.com>

	PR target/93300
	* config/arm/arm.c (arm_block_arith_comp_libfuncs_for_mode): New.
	(arm_init_libfuncs): Add BFmode support to block spurious BF libfuncs.
	Use arm_block_arith_comp_libfuncs_for_mode for HFmode.