main/grub: fix x86_64 grub build problems due to -fno-plt
The grub-bios and grub-xenhost_pvh subpackages fail to build on x86_64 since "-fno-plt" was added to the abuild package:
However grub has not been (re)built since then and so no-one noticed the problem.
Modify the Grub APKBUILD to remove "-fno-plt" when building the grub-bios and grub-xenhost_pvh subpackages.