package/mesa3d: fix build on m68k

package/mesa3d: fix build on m68k

mesa3d uses very big switch statements, which causes the build to fail
on m68k, beause the offsets there are only 16-bit.

We fix that by using -mlong-jump-table-offsets on m68k, to use 32-bit
offsets for switch statements, but this is only available starting with
gcc 7 [0] [1].

Fixes:
http://autobuild.buildroot.net/results/60c4653c2a93125edbdd0beb43cd47301643464a/

Note: we have two packages that select mesa3d, but:
    package/intel-mediadriver/
        -> already depends on x86_64, so implies !m68k

    package/x11r7/xdriver_xf86-video-imx-viv/
        -> imx is an ARM, but xdriver_xf86-video-imx-viv is missing
           a depends on BR2_arm (although the comments do have that
           dependency). However, it depends on other imx related
           packages, and they depend on either arm or aarch64, so
           that implies !m68k.

As such, we do not need to propagate that new dependency.

[0] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57583#c15
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57583#c16

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
[yann.morin.1998@free.fr:
  - add comment
  - reword commit log, add BZ references, add non-propagation notes
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>