Replace BZERO_P/PIC with USE_AS_BZERO/SHARED

Replace BZERO_P with USE_AS_BZERO in i586/i686 memset.S to support i386
multi-arch memset.  Also we should check SHARED not PIC for libc.so
since libc.a may be compiled with PIC.

	* sysdeps/i386/i586/bzero.S (USE_AS_BZERO): New.
	* sysdeps/i386/i686/bzero.S (USE_AS_BZERO): Likewise.
	* sysdeps/i386/i586/memset.S (BZERO_P): Removed.
	Check USE_AS_BZERO/SHARED instead of BZERO_P/PIC.
	(__memset_zero_constant_len_parameter): New.
	* sysdeps/i386/i686/memset.S (BZERO_P): Removed.
	Check USE_AS_BZERO/SHARED instead of BZERO_P/PIC.
	(__memset_zero_constant_len_parameter): Don't define if
	__memset_chk or USE_AS_BZERO are defined.