[gdb/testsuite] Handle unrecognized command line option in gdb_compile_test

[gdb/testsuite] Handle unrecognized command line option in gdb_compile_test

When running the gdb testsuite with gnatmake-4.8, I get many fails of the
following form:
...
gcc: error: unrecognized command line option '-fgnat-encodings=all'^M
gnatmake: "gdb.ada/O2_float_param/foo.adb" compilation error^M
compiler exited with status 1
compilation failed: gcc ... gdb.ada/O2_float_param/foo.adb
gcc: error: unrecognized command line option '-fgnat-encodings=all'
gnatmake: "gdb.ada/O2_float_param/foo.adb" compilation error
FAIL: gdb.ada/O2_float_param.exp: scenario=all: compilation foo.adb
...

Fix this by marking the test unsupported instead, such that we have:
...
UNSUPPORTED: gdb.ada/O2_float_param.exp: scenario=all: compilation foo.adb \
  (unsupported option '-fgnat-encodings=all')
...

Tested on x86_64-linux.