grub: disable build on armv7ve/a with hardfp

The commit(75dbdea9 grub: Allow build on armv7ve/a with softfp)
enable build on armv7ve/a with softfp, but it acutally enable
build on armv7ve/a with hardfp altogether and result in below build
failure:
 | checking for compile options to get strict alignment... -mno-unaligned-access
 | checking if compiler generates unaligned accesses... no
 | checking if C symbols get an underscore after compilation... no
 | checking whether target compiler is working... no
 | configure: error: cannot compile for the target

So update the check to disable build on armv7ve/a with hardfp.

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>