ld.so: Replace (&bootstrap_map) with BOOTSTRAP_MAP

(&_dl_main_map) is used instead of (&bootstrap_map) to bootstrap static
PIE.  Define BOOTSTRAP_MAP with (&_dl_main_map) to avoid hardcode to
(&bootstrap_map).

	* elf/rtld.c (BOOTSTRAP_MAP): New.
	(RESOLVE_MAP): Replace (&bootstrap_map) with BOOTSTRAP_MAP.
	* sysdeps/hppa/dl-machine.h (ELF_MACHINE_BEFORE_RTLD_RELOC):
	Likewise.
	* sysdeps/ia64/dl-machine.h (ELF_MACHINE_BEFORE_RTLD_RELOC):
	Likewise.
	* sysdeps/mips/dl-machine.h (ELF_MACHINE_BEFORE_RTLD_RELOC):
	Likewise.