Add workaround from gdbserver's gdb_proc_service.h to GDB

This commit adds a workaround from gdbserver's gdb_proc_service.h
to GDB's.  It doesn't seem to have been needed on any glibc as far
back as 2001, but it's possibly required for other C libraries so
I've retained it.

gdb/ChangeLog:

	* configure.ac: Check if sys/procfs.h defines elf_fpregset_t.
	(AC_CHECK_HEADERS): Check for linux/elf.h.
	* configure, config.in: Rebuild.
	* gdb_proc_service.h: Include linux/elf.h if sys/procfs.h
	doesn't define elf_fpregset_t.