gdb: testsuite: print explicit test result for gdb.base/dfp-test.exp

gdb: testsuite: print explicit test result for gdb.base/dfp-test.exp

In the current code, if decimal floating point is not supported for
this target, there is no binary file dfp-test, and also there is no
test result after execute the following commands:

  $ make check-gdb TESTS="gdb.base/dfp-test.exp"
  $ grep error gdb/testsuite/gdb.log
  /home/loongson/gdb.git/gdb/testsuite/gdb.base/dfp-test.c:39:1: error: decimal floating point not supported for this target
  [...]
  $ cat gdb/testsuite/gdb.sum
  [...]
  Running target unix
  Running /home/loongson/gdb.git/gdb/testsuite/gdb.base/dfp-test.exp ...

		  === gdb Summary ===
  [...]

With this patch:

  $ make check-gdb TESTS="gdb.base/dfp-test.exp"
  $ cat gdb/testsuite/gdb.sum
  [...]
  Running target unix
  Running /home/loongson/gdb.git/gdb/testsuite/gdb.base/dfp-test.exp ...
  UNSUPPORTED: gdb.base/dfp-test.exp: decimal floating point not supported for this target.

		  === gdb Summary ===

  # of unsupported tests		1
  [...]

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>