gdb/testsuite: extend styling test for libopcodes styling

gdb/testsuite: extend styling test for libopcodes styling

This commit extends the gdb.base/style.exp test to cover disassembler
styling using libopcodes (where available).

The test will try to enable libopcode based styling, if this
works (because such styling is available) then some tests are run to
check that the output is styled, and that the styling can be disabled
using 'set style disassembler enabled off'.  If libopcodes styling is
not available on the current architecture then these new tests will be
skipped.

I've moved the Python Pygments module check inside the
test_disable_disassembler_styling function now, so that the test will
be run even when Python Pygments is not available, this allows the new
tests discussed above.

If the Pygments module is not available then the Pygments based tests
will be skipped just as they were before.