Extend Encode_XSEncoding for the symbol name

Store the symbol name along the IV ptr as SVp_POK PVX.
This is needed for perlcc when thawing frozen images, where an
encoding object was initialized at compile-time, pointing to
an extern symbol of the shared library, and need to be remapped
to the new ptr at run-time.
See e.g. https://rt.cpan.org/Public/Bug/Display.html?id=94069
where Net::DNS uses an encoding via use constant, causing a segfault,
and https://code.google.com/p/perl-compiler/issues/detail?id=305 for
the perlcc support code.
Which is something along:

  #include <dlfcn.h>
  void *handle = dlopen(sofile, RTLD_NOW|RTLD_NOLOAD);
  void *ascii_encoding = dlsym(handle, "ascii_encoding");
  SvIV_set(&sv_list[1], PTR2IV(ascii_encoding));

Detection:
  https://github.com/rurban/perl-compiler/blob/ce6ce452dd8fcc9bb90605dcf47489250fd540a1/lib/B/C.pm#L2545
Remapping:
  https://github.com/rurban/perl-compiler/blob/ce6ce452dd8fcc9bb90605dcf47489250fd540a1/lib/B/C.pm#L4851
fix -Wdeclaration-after-statement for ISO C90

Tested ok on all major perl versions:
$ perlall -mq maketest
$ grep Result: log.test-linuxmint1-5.*
log.test-linuxmint1-5.10.1d:Result: PASS
log.test-linuxmint1-5.10.1-nt:Result: PASS
log.test-linuxmint1-5.12.5:Result: PASS
log.test-linuxmint1-5.12.5-nt:Result: PASS
log.test-linuxmint1-5.14.4:Result: PASS
log.test-linuxmint1-5.14.4-nt:Result: PASS
log.test-linuxmint1-5.16.3:Result: PASS
log.test-linuxmint1-5.16.3-nt:Result: PASS
log.test-linuxmint1-5.18.2:Result: PASS
log.test-linuxmint1-5.18.2-nt:Result: PASS
log.test-linuxmint1-5.8.4d-nt:Result: PASS
log.test-linuxmint1-5.8.5-nt:Result: PASS
log.test-linuxmint1-5.8.8d-nt:Result: PASS
log.test-linuxmint1-5.8.9:Result: PASS
log.test-linuxmint1-5.8.9-nt:Result: PASS

In particular it is safe not to use a heap copy of the PVX
since it's a SVp_POK with LEN=0, which is never freed, thus freed
along the shared library.
Merge pull request #20 from rurban/remap-symname

Remap symname [RT #94221]