i386: Add <startup.h> [BZ #21913]hjl/pr21913/master

On Linux/i386, there are 3 ways to make a system call:

1. call *%gs:SYSINFO_OFFSET.  This requires TLS initialization.
2. call *_dl_sysinfo.  This requires relocation of _dl_sysinfo.
3. int $0x80.  This is slower than #2 and #3, but works everywhere.

When an object file is compiled with PIC, #1 is prefered since it is
faster than #3 and doesn't require relocation of _dl_sysinfo.  For
dynamic executables, ld.so initializes TLS.  However, for static
executables, before TLS is initialized by __libc_setup_tls, #3 should
be used for syscalls.

This patch adds <startup.h> which defines _startup_fatal and defaults
it to __libc_fatal.  It replaces __libc_fatal with _startup_fatal in
static executables where it is called before __libc_setup_tls is called.
This header file is included in all files containing functions which are
called before __libc_setup_tls is called.  On Linux/i386, when PIE is
enabled by default, _startup_fatal is turned into ABORT_INSTRUCTION and
I386_USE_SYSENTER is defined to 0 so that "int $0x80" is used for system
calls before __libc_setup_tls is called.

	[BZ #21913]
	* config.h.in (BUILD_PIE_DEFAULT): New.
	* csu/libc-tls.c: Include <startup.h>.
	* elf/dl-tunables.c: Likewise.
	* sysdeps/unix/sysv/linux/i386/brk.c: Likewise.
	* csu/libc-tls.c: Include <startup.h>.
	(__libc_setup_tls): Call _startup_fatal instead of __libc_fatal.
	* sysdeps/generic/startup.h: New file.
	* sysdeps/unix/sysv/linux/i386/startup.h: Likewise.

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