gdb: build solib-svr4-linux.o when building for targets xtensa*-*-*linux*

gdb: build solib-svr4-linux.o when building for targets xtensa*-*-*linux*

When building a GDB configured with --target=xtensa-buildroot-linux-uclibc, we get:

  CXXLD  gdb
/usr/bin/ld: xtensa-linux-tdep.o: in function `xtensa_linux_init_abi(gdbarch_info, gdbarch*)':
/home/simark/src/binutils-gdb/gdb/xtensa-linux-tdep.c:115:(.text+0x102): undefined reference to `make_linux_ilp32_svr4_solib_ops(program_space*)'

Fix it by adding solib-svr4-linux.o to the list of objects build for
targets xtensa*-*-*linux*.

Change-Id: I0caa04a1d1c097defb5ea9c285cccc09ba80058d