gdb: Initial baremetal riscv support

This commit introduces basic support for baremetal RiscV as a GDB
target.  This target is currently only tested against the RiscV software
simulator, which is not included as part of this commit.  The target has
been tested against the following RiscV variants: rv32im, rv32imc,
rv32imf, rv32imfc, rv64im, rv64imc, rv64imfd, rv64imfdc.

Across these variants we pass on average 34858 tests, and fail 272
tests, which is ~0.8%.

The RiscV has a feature of its ABI where structures with a single
floating point field, a single complex float field, or one float and
one integer field are treated differently for argument passing.  The
new test gdb.base/infcall-nested-structs.exp is added to cover this
feature.  As passing these structures should work on all targets then
I've made the test as a generic one, even though, for most targets,
there's probably nothing special about any of these cases.

gdb/ChangeLog:

	* Makefile.in (ALL_TARGET_OBS): Add riscv-tdep.o
	(HFILES_NO_SRCDIR): Add riscv-tdep.h.
	(ALLDEPFILES): Add riscv-tdep.c
	* configure.tgt: Add riscv support.
	* riscv-tdep.c: New file.
	* riscv-tdep.h: New file.
	* NEWS: Mention new target.
	* MAINTAINERS: Add entry for riscv.

gdb/testsuite/ChangeLog:

	* gdb.base/infcall-nested-structs.exp: New file.
	* gdb.base/infcall-nested-structs.c: New file.
	* gdb.base/float.exp: Add riscv support.

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