[build] Use unique entry symbols for each prefix

Some binutils versions will drag in an object to satisfy the entry
symbol; some won't.  Try to cope with this exciting variety of
behaviour by ensuring that all entry symbols are unique.

Remove the explicit inclusion of the prefix object on the linker
command line, since the entry symbol now provides all the information
needed to identify the prefix.

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

src/arch/x86_64/prefix/linuxprefix.S

@@ -2,10 +2,10 @@
 
 	.section ".text"
 	.code64
-	.globl _start
-	.type _start, @function
+	.globl _linux_start
+	.type _linux_start, @function
 
-_start:
+_linux_start:
 	xorq	%rbp, %rbp
 
 	popq	%rdi       // argc -> C arg1

src/arch/x86_64/scripts/linux.lds

@@ -8,8 +8,6 @@
 OUTPUT_FORMAT ( "elf64-x86-64", "elf64-x86-64", "elf64-x86-64" )
 OUTPUT_ARCH ( i386:x86-64 )
 
-ENTRY ( _start )
-
 SECTIONS {
 	_max_align = 32;