configure: make more robust the check for elf 64

configure: make more robust the check for elf 64

The check if 'elf.h' defines the type 'Elf64_Ehdr' is currently
done through 'grep' on the file. While there is no false positive,
so far, such test could incorrectly find the text inside a comment
or in a block guarded by #if/#endif.

Use the autoconf macro AC_CHECK_TYPE() to detect if the type is
properly declared.

Change-Id: Ibb74db3d90ac6d1589b9dc1e5a7ae59e47945e78
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8591
Tested-by: jenkins