Print bfloat16 DWARF types correctly

Print bfloat16 DWARF types correctly

Even if the DWARF information contains a bfloat16 base type (__bf16), a
variable of such type will still be printed using the IEEE half float format,
which is wrong.

This patch teaches GDB how to pick the bfloat16 format for __bf16 types in
DWARF (based on the base type name) and uses IEEE half float for all the other
16-bit float formats.

Tested on aarch64-linux/x86_64-linux.

OK?

gdb/ChangeLog:

2021-04-16  Luis Machado  <luis.machado@linaro.org>

	* arch-utils.c (default_floatformat_for_type): Handle bfloat16.

gdb/testsuite:

2021-04-16  Luis Machado  <luis.machado@linaro.org>

	* gdb.dwarf2/dw2-bfloat16.exp: New file.