kernel/generic: fix the arm gc-sections patch

kernel/generic: fix the arm gc-sections patch

Latest binutils (2.37) exposed a long-standing bug. The kernel linking stage
would break at the SORTTAB step, due to the exception table having been
previously purged from vmlinux, as its section wasn't marked as unconditionally
kept. Fix thusly.

Additionally, the "#define ARM_MMU_DISCARD(x) KEEP(x)" change is bogus. It
would only apply to !CONFIG_MMU devices (which we don't support in OpenWrt), and
it would even break the build if referenced. Drop it.

While at it, rename the patch in order to make it obvious that it's
arm-specific.

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
[Add same changes for kernel 5.4 too]
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>