gdb/riscv: select rv32 target by default when requested

gdb/riscv: select rv32 target by default when requested

GDB for RISC-V always uses target descriptions.  When the target
doesn't provide a target description then a default is selected.
Usually this default is selected based on the properties of the
executable being debugged.  However, when there is no executable being
debugged we currently fallback to the riscv:rv64 target description as
the default.  This leads to strange behaviour like this:

  $ gdb
  (gdb) set architecture riscv:rv32
  (gdb) p sizeof ($pc)
  $1 = 8

Despite the users specifically setting the architecture to riscv:rv32
GDB still thinks that the target has riscv:rv64 register sizes.

The above is a bit of a contrived situation.  I actually ran into this
situation while trying to connect to a running riscv:rv32 target
without supplying an executable (the target didn't provide a target
description).  When I tried to set a register on the target I ran into
errors because GDB was passing 8 bytes to the target rather than the
expected 4.  Even when I manually specified the architecture (as
above) I couldn't convince GDB to only send 4 bytes.

This patch fixes this issue.  Now, when we selected a default target
description we will make use of the user selected architecture to
guide our choice.  In the above example we now get:

  $ gdb
  (gdb) set architecture riscv:rv32
  (gdb) p sizeof ($pc)
  $1 = 4

And my real world example of connecting to a remote without an
executable works fine.

I've used the fact that we can ask GDB about $pc even when no
executable is loaded as the basis for a test to cover this situation.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_features_from_gdbarch_info): Rename to...
	(riscv_features_from_bfd): ...this.  Change parameter type to
	'bfd*', and update as required.
	(riscv_find_default_target_description): Update call to
	riscv_features_from_bfd.  Select a default xlen based on
	info.bfd_arch_info.
	(riscv_gdbarch_init): Update call to riscv_features_from_bfd.

gdb/testsuite/ChangeLog:

	* gdb.arch/riscv-default-tdesc.exp: New file.