main/grub: fix x86_64 grub build problems due to -fno-plt

Merged Dermot Bradley requested to merge dbradley/aports:fix-grub-build into master

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.

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