gdb/riscv: Update API for looking up target descriptions

gdb/riscv: Update API for looking up target descriptions

In preparation for adding the RISC-V gdbserver, this commit
restructures the API for looking up target descriptions.

The current API is riscv_create_target_description, which creates a
target description from a riscv_gdbarch_features, but also caches the
created target descriptions so that for a given features object we
always get back the same target description object.  This is important
for GDB due to the way gdbarch objects are reused.

As the same target description is always returned to GDB, and can be
returned multiple times, it is returned as a const, however, the
current cache actually stores a non-const target description.  This is
improved in this patch so that the cache holds a const target
description.

For gdbsever, this caching of the target descriptions is not needed,
the gdbserver looks up one target description to describe the target
it is actually running on and that is it.  Further the gdbserver
actually needs to modify the target description that is looked up, so
for the gdbsever, returning a const target description is not
acceptable.

This commit aims to address this by creating two parallel target
description APIs, on is the old riscv_create_target_description,
however, this no longer performs any caching, and just creates a new
target description, and returns it as non-const.

The second API is riscv_lookup_target_description, this one performs
the caching, and calls riscv_create_target_description to create a
target description when needed.

In order to make sure the correct API is used in the correct place I
have guarded the code using the GDBSERVER define.  For GDB the
riscv_create_target_description is static, and not generally usable
throughout GDB, only the lookup API is global.  In gdbserver, the
lookup functions, and the cache are not defined or created at all,
only the riscv_create_target_description API is available.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* arch/riscv.c (struct riscv_gdbarch_features_hasher): Only define
	if GDBSERVER is not defined.
	(riscv_tdesc_cache): Likewise, also store const target_desc.
	(STATIC_IN_GDB): Define.
	(riscv_create_target_description): Update declaration with
	STATIC_IN_GDB.
	(riscv_lookup_target_description): New function, only define if
	GDBSERVER is not defined.
	* arch/riscv.h (riscv_create_target_description): Declare only
	when GDBSERVER is defined.
	(riscv_lookup_target_description): New declaration when GDBSERVER
	is not defined.
	* nat/riscv-linux-tdesc.c (riscv_linux_read_description): Rename to...
	(riscv_linux_read_features): ...this, and return
	riscv_gdbarch_features instead of target_desc.
	* nat/riscv-linux-tdesc.h: Include 'arch/riscv.h'.
	(riscv_linux_read_description): Rename to...
	(riscv_linux_read_features): ...this.
	* riscv-linux-nat.c (riscv_linux_nat_target::read_description):
	Update to use riscv_gdbarch_features and
	riscv_lookup_target_description.
	* riscv-tdep.c (riscv_find_default_target_description): Use
	riscv_lookup_target_description instead of
	riscv_create_target_description.