libopcodes/s390: add support for disassembler styling

This commit adds disassembler style to the libopcodes s390
disassembler.  This conversion was pretty straight forward, I just
converted the fprintf_func calls to fprintf_styled_func calls and
added an appropriate style.

For testing the new styling I just assembled then disassembled the
source files in gas/testsuite/gas/s390 and manually checked that the
styling looked reasonable.

If the user does not request styled output from objdump, then there
should be no change in the disassembler output after this commit.