Cleanup GPL header address notices by using http://www.gnu.org/licenses/

Cleanup GPL header address notices by using gnu.org/licenses/

Sync VEX/LICENSE.GPL with top-level COPYING file. We used 3 different
addresses for writing to the FSF to receive a copy of the GPL. Replace
all different variants with an URL <http://www.gnu.org/licenses/>.

The following files might still have some slightly different (L)GPL
copyright notice because they were derived from other programs:

- files under coregrind/m_demangle which come from libiberty:
  cplus-dem.c, d-demangle.c, demangle.h, rust-demangle.c,
  safe-ctype.c and safe-ctype.h
- coregrind/m_demangle/dyn-string.[hc] derived from GCC.
- coregrind/m_demangle/ansidecl.h derived from glibc.
- VEX files for FMA detived from glibc:
  host_generic_maddf.h and host_generic_maddf.c
- files under coregrin/m_debuginfo derived from LZO:
  lzoconf.h, lzodefs.h, minilzo-inl.c and minilzo.h
- files under coregrind/m_gdbserver detived from GDB:
  gdb/signals.h, inferiors.c, regcache.c, regcache.h,
  regdef.h, remote-utils.c, server.c, server.h, signals.c,
  target.c, target.h and utils.c

Plus the following test files:

- none/tests/ppc32/testVMX.c derived from testVMX.
- ppc tests derived from QEMU: jm-insns.c, ppc64_helpers.h
  and test_isa_3_0.c
- tests derived from bzip2 (with embedded GPL text in code):
  hackedbz2.c, origin5-bz2.c, varinfo6.c
- tests detived from glibc: str_tester.c, pth_atfork1.c
- test detived from GCC libgomp: tc17_sembar.c
- performance tests derived from bzip2 or tinycc (with embedded GPL
  text in code): bz2.c, test_input_for_tinycc.c and tinycc.c