[linux] Use host glibc system call wrappers

When building as a Linux userspace application, iPXE currently
implements its own system calls to the host kernel rather than relying
on the host's C library.  The output binary is statically linked and
has no external dependencies.

This matches the general philosophy of other platforms on which iPXE
runs, since there are no external libraries available on either BIOS
or UEFI bare metal.  However, it would be useful for the Linux
userspace application to be able to link against host libraries such
as libslirp.

Modify the build process to perform a two-stage link: first picking
out the requested objects in the usual way from blib.a but with
relocations left present, then linking again with a helper object to
create a standard hosted application.  The helper object provides the
standard main() entry point and wrappers for the Linux system calls
required by the iPXE Linux drivers and interface code.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86_64/Makefile.linux

@@ -1,6 +1,10 @@
+# -*- makefile -*- : Force emacs to use Makefile mode
+
+# Linker script
+#
 LDSCRIPT = arch/x86_64/scripts/linux.lds
 
-SRCDIRS += arch/x86_64/core/linux
-
+# Include generic Linux Makefile
+#
 MAKEDEPS += arch/x86/Makefile.linux
 include arch/x86/Makefile.linux

src/arch/x86_64/core/linux/linux_syscall.S

@@ -1,33 +0,0 @@
-
-	.section ".data"
-	.globl linux_errno
-
-linux_errno:	.int 0
-
-	.section ".text"
-	.code64
-	.globl linux_syscall
-	.type  linux_syscall, @function
-
-linux_syscall:
-	movq	%rdi, %rax    // C arg1 -> syscall number
-	movq	%rsi, %rdi    // C arg2 -> syscall arg1
-	movq	%rdx, %rsi    // C arg3 -> syscall arg2
-	movq	%rcx, %rdx    // C arg4 -> syscall arg3
-	movq	%r8, %r10     // C arg5 -> syscall arg4
-	movq	%r9, %r8      // C arg6 -> syscall arg5
-	movq	8(%rsp), %r9  // C arg7 -> syscall arg6
-
-	syscall
-
-	cmpq	$-4095, %rax
-	jae	1f
-	ret
-
-1:
-	negq	%rax
-	movl	%eax, linux_errno
-	movq	$-1, %rax
-	ret
-
-	.size linux_syscall, . - linux_syscall

src/arch/x86_64/core/linux/linuxprefix.S

@@ -1,25 +0,0 @@
-#include <linux/unistd.h>
-
-	.section ".text"
-	.code64
-	.globl _linux_start
-	.type _linux_start, @function
-
-_linux_start:
-	xorq	%rbp, %rbp
-
-	popq	%rdi       // argc -> C arg1
-	movq	%rsp, %rsi // argv -> C arg2
-
-	andq	$~15, %rsp // 16-byte align the stack
-
-	call	save_args
-
-	/* Our main doesn't use any arguments */
-	call	main
-
-	movq	%rax, %rdi // rc -> syscall arg1
-	movq	$__NR_exit, %rax
-	syscall
-
-	.size _linux_start, . - _linux_start

src/arch/x86_64/include/bits/linux_api.h

@@ -1,6 +0,0 @@
-#ifndef _X86_64_LINUX_API_H
-#define _X86_64_LINUX_API_H
-
-#define __SYSCALL_mmap __NR_mmap
-
-#endif /* _X86_64_LINUX_API_H */