Update GDB gdb_proc_service.h workaround to match gdbserver

This commit updates GDB's gdb_proc_service.h to use elf_{g,fp}regset_t
instead of gdb_{g,fp}regset_t if pr{g,fp}regset_t are undefined.
The types have been equivalent on GNU/Linux since at least 2005.

gdb/ChangeLog:

	* gdb_proc_service.h: Use elf_gregset_t if prgregset_t is
	undefined.  Use elf_fpregset_t if prfpregset_t is undefined.