buildgcc: Match the string of downloading percentage more precisely

The command "wget" prints some hyperlink with "%", which will be
filtered in by previous regular expression. So we need to change to
match the string with exactly 3 digits and a percent symbol.

TEST:
echo 45%  | grep -o "\<[0-9]\{1,3\}%"
  45%
echo 1245% | grep -o "\<[0-9]\{1,3\}%"
  <empty>
echo aa%  | grep -o "\<[0-9]\{1,3\}%"
  <empty>

Change-Id: I6ef9e7c87fd4ee6cc707346954d91e6e3af3b939
Signed-off-by: Zheng Bao <fishbaozi@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66743
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>

util/crossgcc/buildgcc

@@ -1076,7 +1076,7 @@ if searchtool wget "GNU" nofail > /dev/null; then
 		url=$1
 		printf "... ${red}  0%%"
 		wget --tries=3 "$url" 2>&1 | while read -r line; do
-			echo "$line" | grep -o "[0-9]\+%" | awk '{printf("\b\b\b\b%4s", $1)}'
+			echo "$line" | grep -o "\<[0-9]\{1,3\}%" | awk '{printf("\b\b\b\b%4s", $1)}'
 		done
 		printf "${NC}... "
 	}