kbuild: fix buglet in gcc-version.sh

Greg Schafer <gschafer@zip.com.au> reported:
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$make mrproper
scripts/gcc-version.sh: [[: command not found

This is on a very old host with an ancient bash as /bin/sh. But I have
CONFIG_SHELL set and pointing to a modern bash. Something is wrong.

This doesn't happen with 2.6.23
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Fixed using a more common string equality test.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Greg Schafer <gschafer@zip.com.au>
Cc: Jesper Juhl <jesper.juhl@gmail.com>