gdb/testsuite: fix buffer overflow in gdb.base/signed-builtin-types.exp

gdb/testsuite: fix buffer overflow in gdb.base/signed-builtin-types.exp

In commit:

  commit 9f50fe0835850645bd8ea9bb1efe1fe6c48dfb12
  Date:   Wed Dec 7 15:55:25 2022 +0000

      gdb/testsuite: new test for recent dwarf reader issue

A new test (gdb.base/signed-builtin-types.exp) was added that made use
of 'info sources' to figure out if the debug information for a
particular object file had been fully expanded or not.  Unfortunately
some lines of the 'info sources' output can be very long, this was
observed on some systems where the debug information for the
dynamic-linker was installed, in this case, the list of source files
associated with the dynamic linker was so long it would cause expect's
internal buffer to overflow.

This commit switches from using 'info sources' to 'maint print
objfile', the output from the latter command is more compact, but
also, can be restricted to a single named object file.

With this change in place I am no longer seeing buffer overflow errors
from expect when running gdb.base/signed-builtin-types.exp.