scripts: Create objdump-func helper script

Add a simple script which disassembles a single function from an object
file.  Comes in handy for objtool warnings and kernel stack traces.
Originally-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/3eb3f091fd6bd9caba50392ceab98ce756804f3b.1650578171.git.jpoimboe@redhat.com

scripts/objdump-func

+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Disassemble a single function.
+#
+# usage: objdump-func <file> <func>
+
+set -o errexit
+set -o nounset
+
+OBJDUMP="${CROSS_COMPILE:-}objdump"
+
+command -v gawk >/dev/null 2>&1 || die "gawk isn't installed"
+
+usage() {
+	echo "usage: objdump-func <file> <func>" >&2
+	exit 1
+}
+
+[[ $# -lt 2 ]] && usage
+
+OBJ=$1; shift
+FUNC=$1; shift
+
+# Secret feature to allow adding extra objdump args at the end
+EXTRA_ARGS=$@
+
+# Note this also matches compiler-added suffixes like ".cold", etc
+${OBJDUMP} -wdr $EXTRA_ARGS $OBJ | gawk -M -v f=$FUNC '/^$/ { P=0; } $0 ~ "<" f "(\\..*)?>:" { P=1; O=strtonum("0x" $1); } { if (P) { o=strtonum("0x" $1); printf("%04x ", o-O); print $0; } }'