Don't provide a default _start symbol for entry point

This was a nonstandard experiment, to get rid of:
warning: cannot find entry symbol _start; defaulting to 000000a0
when gcc -nostartfiles was used.

Traditionally, when no entry point is specified, the .text section
of the first object on the ld command-line is used as entry.
This is generally what is expected.

But this behaviour is defeated on ELF targets, where the linker script
is more complex. For example, the .text.startup sections are linked
before .text sections.
If a C source file contains a function named main() and is compiled
with -O2, gcc automatically puts it into the .text.startup section.
If the user provided a custom startup code and put it first on the ld
command-line, that will be defeated because main() will actually be
linked first, before the startup code.
This is what happened with the gemma sample programs. They crashed on
exit, at the end of main() as there was no actual startup code.

This explains why that situation is dangerous, and why that standard warning is
useful. So I enabled it again, like standard ELF.

The easy fix to get rid of that warning is simply to add a global _start label
at the beginning of the custom startup code. That will work in any case,
whereever the startup code will actually be located.

Author: vinriviere

ld/scripttempl/m68kmintelf.sc

@@ -43,7 +43,6 @@ SECTIONS
   /* Program code.  */
   .text :
   {
-    PROVIDE(_start = .); /* Default entry point if _start isn't defined.  */
     *crt0.o(.text .text.*)
     *(.text.unlikely .text.*_unlikely .text.unlikely.*)
     *(.text.exit .text.exit.*)