testsuite: solib-disc: Use `standard_output_file'

testsuite: solib-disc: Use `standard_output_file'

Correct a commit 2151ccc56c74 ("Always organize test artifacts in a
directory hierarchy") regression causing:

Running .../gdb/testsuite/gdb.base/solib-disc.exp ...
gdb compile failed, Assembler messages:
Fatal error: can't create .../gdb/testsuite/gdb.base/so-disc-shr.c.o: No such file or directory

by using `standard_output_file' to construct output file names
throughout.

	gdb/testsuite/
	* gdb.base/solib-disc.exp: Use `standard_output_file'
	throughout.