gnu: gcj-4.8, gcj: Update to default GCC version.

* gnu/packages/gcc.scm (gcj-4.8): Inherit from "gcc" package and rename
  variable to...
  (gcj): ...this new variable.
  (ecj-bootstrap-4.8): Rename variable to...
  (ecj-bootstrap): ...this, updating to version 4.9.
* gnu/packages/java.scm (icedtea6)[native-inputs]: Replace "gcj-4.8"
  with "gcj".
* gnu/packages/music.scm (tuxguitar)[native-inputs]: Likewise.