Fix wrong macro name in FreeBSD config for RISCV

[Kartatz]

The author of 48abb54 misspelled the macro name in the FreeBSD config file for RISCV. This unfortunately causes the compilation of GCC toolchains targeting riscv64-unknown-freebsd14.* to fail with the following error:

In file included from ./tm.h:44,
                 from ../../gcc/gcc.cc:35:
../../gcc/config/riscv/freebsd.h:45:5: error: expected ‘,’ or ‘;’ before ‘FBSD_LINK_PG_NOTES’
   45 |   " FBSD_LINK_PG_NOTES "                                                \
      |     ^~~~~~~~~~~~~~~~~~
../../gcc/gcc.cc:1211:32: note: in expansion of macro ‘LINK_SPEC’
 1211 | static const char *link_spec = LINK_SPEC;
      |                                ^~~~~~~~~