Fixes for the magic number used in PDP11 AOUT binaries.

PR ld/25677
include	* aout/aout64.h (N_DATADDR): Add IMAGIC case.

bfd	* pdp11.c: Add implementation of --imagic option.
	(adjust_o_magic): Fix objcopy --extract-symbol test.
	* libaout.h (enum aout_magic): Add i_magic.

ld	* emulparams/pdp11.sh (SCRIPT_NAME): Change to pdp11.
	(EXTRA_EM_FILE): New, add emulation file pdp11.
	* scripttempl/pdp11.sc: New, derived from aout.sc without
	irrelevant input sections.
	* emultempl/pdp11.em (_add_options, _handle_option)
	(_list_options): New. Add options -z, --imagic for pdp11-aout.
	(_before_parse): Make --omagic be default instead of --nmagic.
	(_get_script): Modify special-case linker script for --imagic.
	* lexsup.c (parse_args): Explictly set config.text_read_only for -n.
	* ld.texi (Options): Add documentation of PDP11-specific options.
	(Options): Fix unrelated typo to --no-compact-branches.
	* gen-doc.texi: @set PDP11.
	* testsuite/ld-pdp11/pdp11.exp: New, start pdp11 testing.
	* testsuite/ld-pdp11/sections.s: New, source for options tests.
	* testsuite/ld-pdp11/imagic.d: New, test --imagic format.
	* testsuite/ld-pdp11/imagicz.d: New, test -z (imagic) format.
	* testsuite/ld-pdp11/nmagic.d: New, test --nmagic format.
	* testsuite/ld-pdp11/omagic.d: New, test --omagic format.

ld/scripttempl/pdp11.sc

@@ -0,0 +1,56 @@
+# Copyright (C) 2014-2020 Free Software Foundation, Inc.
+#
+# Copying and distribution of this file, with or without modification,
+# are permitted in any medium without royalty provided the copyright
+# notice and this notice are preserved.
+#
+test -z "${BIG_OUTPUT_FORMAT}" && BIG_OUTPUT_FORMAT=${OUTPUT_FORMAT}
+test -z "${LITTLE_OUTPUT_FORMAT}" && LITTLE_OUTPUT_FORMAT=${OUTPUT_FORMAT}
+test -z "${ALIGNMENT}" && ALIGNMENT="2"
+
+cat <<EOF
+/* Copyright (C) 2014-2020 Free Software Foundation, Inc.
+
+   Copying and distribution of this script, with or without modification,
+   are permitted in any medium without royalty provided the copyright
+   notice and this notice are preserved.  */
+
+OUTPUT_FORMAT("${OUTPUT_FORMAT}", "${BIG_OUTPUT_FORMAT}",
+	      "${LITTLE_OUTPUT_FORMAT}")
+OUTPUT_ARCH(${ARCH})
+
+${RELOCATING+${LIB_SEARCH_DIRS}}
+${STACKZERO+${RELOCATING+${STACKZERO}}}
+${SHLIB_PATH+${RELOCATING+${SHLIB_PATH}}}
+${RELOCATING+${EXECUTABLE_SYMBOLS}}
+${RELOCATING+PROVIDE (__stack = 0);}
+SECTIONS
+{
+  ${RELOCATING+. = ${TEXT_START_ADDR};}
+  .text :
+  {
+    CREATE_OBJECT_SYMBOLS
+    *(.text)
+    ${RELOCATING+_etext = .;}
+    ${RELOCATING+__etext = .;}
+    ${PAD_TEXT+${RELOCATING+. = ${DATA_ALIGNMENT};}}
+  }
+  ${RELOCATING+. = ${DATA_ALIGNMENT};}
+  .data :
+  {
+    *(.data)
+    ${CONSTRUCTING+CONSTRUCTORS}
+    ${RELOCATING+_edata  =  .;}
+    ${RELOCATING+__edata  =  .;}
+  }
+  .bss :
+  {
+   ${RELOCATING+ __bss_start = .};
+   *(.bss)
+   *(COMMON)
+   ${RELOCATING+. = ALIGN(${ALIGNMENT});}
+   ${RELOCATING+_end = . };
+   ${RELOCATING+__end = . };
+  }
+}
+EOF