Don't include *sol2-tdep.o on Linux/sparc*

Don't include *sol2-tdep.o on Linux/sparc*

Linux/sparc* currently links Solaris-specific files (sparc-sol2-tdep.o,
sparc64-sol2-tdep.o, sol2-tdep.o) for no apparent reason.  It has no
business doing so, and none of the functions/variables defined there are
used explicitly.  If support for the Solaris OSABI were desired, this
should be done using --enable-targets instead.

Since neither sparc{32,64}_sol2_init_abi currently declared in common
headers (sparc*-tdep.h) are used outside their source files, they are made
static and the declarations removed.

Tested on sparcv9-sun-solaris2.11 and sparc64-unknown-linux-gnu.

	* configure.tgt <sparc-*-linux*> (gdb_target_obs): Remove
	sparc-sol2-tdep.o, sol2-tdep.o, sparc64-sol2-tdep.o.
	<sparc64-*-linux*> (gdb_target_obs): Remove sparc64-sol2-tdep.o,
	sol2-tdep.o, sparc-sol2-tdep.o.
	* sparc-sol2-tdep.c (sparc32_sol2_init_abi): Make static.
	* sparc-tdep.h (sparc32_sol2_init_abi): Remove.
	* sparc64-sol2-tdep.c (sparc64_sol2_init_abi): Make static.
	* sparc64-tdep.h (sparc64_sol2_init_abi): Remove.