Add initial compile command support to RISC-V port.

Add initial compile command support to RISC-V port.

This adds initial compile command support to the RISC-V port.  This fixes
about 228 testsuite failures on a riscv64-linux machine.  We need to get
the triplet right which is normally riscv64 or riscv32 instead of the
default riscv.  Also, we need to get the compiler options right, since we
don't accept the default -m64 and -mcmodel=large options, so we need to
construct -march and -mabi options which are correct for the target.  We
currently don't have info about all extensions used by the target, so this
may need to be adjusted later.  For now, I'm assuming that we have all
extensions required by the linux platform spec.

	gdb/
	* riscv-tdep.c (riscv_gcc_target_options): New.
	(riscv_gnu_triplet_regexp): New.
	(riscv_gdbarch_init): Call set_gdbarch_gcc_triplet_options and
	set_gdbarch_gnu_triplet_regexp.

Change-Id: I315ce8de7789ddf7bdd3b532f917519464941294