x86/realmode: Explicitly set entry point via ENTRY in linker script

Linking with ld.lld via

$ make LD=ld.lld

produces the warning:

  ld.lld: warning: cannot find entry symbol _start; defaulting to 0x1000

Linking with ld.bfd shows the default entry is 0x1000:

$ readelf -h arch/x86/realmode/rm/realmode.elf | grep Entry
  Entry point address:               0x1000

While ld.lld is being pedantic, just set the entry point explicitly,
instead of depending on the implicit default. The symbol pa_text_start
refers to the start of the .text section, which may not be at 0x1000 if
the preceding sections listed in arch/x86/realmode/rm/realmode.lds.S
were large enough. This matches behavior in arch/x86/boot/setup.ld.
Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Suggested-by: Borislav Petkov <bp@alien8.de>
Suggested-by: Peter Smith <Peter.Smith@arm.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: clang-built-linux@googlegroups.com
Cc: grimar@accesssoftek.com
Cc: Ingo Molnar <mingo@redhat.com>
Cc: maskray@google.com
Cc: ruiu@google.com
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190925180908.54260-1-ndesaulniers@google.com
Link: https://github.com/ClangBuiltLinux/linux/issues/216

arch/x86/realmode/rm/realmode.lds.S

@@ -11,6 +11,7 @@
 
 OUTPUT_FORMAT("elf32-i386")
 OUTPUT_ARCH(i386)
+ENTRY(pa_text_start)
 
 SECTIONS
 {