lang/gcc10: fix build with clang 17

When building lang/gcc10 with recent clang versions, which default to
C++17, the following errors are produced:

  cfns.gperf:35:18: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
     35 | libc_name::hash (register const char *str, register unsigned int len)
        |                  ^~~~~~~~
  cfns.gperf:35:44: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
     35 | libc_name::hash (register const char *str, register unsigned int len)
        |                                            ^~~~~~~~
  ...many more of these...

Upstream fixed this in
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=642dc602f89b2c11d5d833a52f37b04427a27cab,
so apply it.