release: Support GNU stat in mkisoimages.sh

BSD stat and GNU stat differ significantly when it comes to using a
custom format string, both in the option name and in the format string
itself. Handle both here (assuming Linux means GNU stat rather than BSD
stat).

Reviewed by:	brooks, gjb
Obtained from:	CheriBSD
Differential Revision:	https://reviews.freebsd.org/D35814

release/amd64/mkisoimages.sh

@@ -112,7 +112,11 @@ if [ "$bootable" != "" ]; then
 
 	# Create a GPT image containing the partitions we need for hybrid boot.
 	hybridfilename=$(mktemp /tmp/hybrid.img.XXXXXX)
-	imgsize=`stat -f %z "$NAME"`
+	if [ "$(uname -s)" = "Linux" ]; then
+		imgsize=`stat -c %s "$NAME"`
+	else
+		imgsize=`stat -f %z "$NAME"`
+	fi
 	$MKIMG -s gpt \
 	    --capacity $imgsize \
 	    -b "$BASEBITSDIR/boot/pmbr" \

release/arm64/mkisoimages.sh

@@ -107,7 +107,11 @@ if [ "$bootable" != "" ]; then
 
 	# Create a GPT image containing the EFI partition.
 	efifilename=$(mktemp /tmp/efi.img.XXXXXX)
-	imgsize=`stat -f %z "$NAME"`
+	if [ "$(uname -s)" = "Linux" ]; then
+		imgsize=`stat -c %s "$NAME"`
+	else
+		imgsize=`stat -f %z "$NAME"`
+	fi
 	$MKIMG -s gpt \
 	    --capacity $imgsize \
 	    $espparam \

release/riscv/mkisoimages.sh

@@ -107,7 +107,11 @@ if [ "$bootable" != "" ]; then
 
 	# Create a GPT image containing the EFI partition.
 	efifilename=$(mktemp /tmp/efi.img.XXXXXX)
-	imgsize=`stat -f %z "$NAME"`
+	if [ "$(uname -s)" = "Linux" ]; then
+		imgsize=`stat -c %s "$NAME"`
+	else
+		imgsize=`stat -f %z "$NAME"`
+	fi
 	$MKIMG -s gpt \
 	    --capacity $imgsize \
 	    $espparam \