rules.mk: use -fPIC instead of -fpic on arm64

rules.mk: use -fPIC instead of -fpic on arm64

Some packages fail to build on arm64 when PKG_ASLR_PIE_ALL=y, due to
machine-specific size restrictions on the global offset table.
While the manual instructs to recompile with -fPIC if it fails with
-fpic, by doing this per package, there is still a risk of random
breakage due to version bumps or other changes, so let's use -fPIC on
arm64 by default.

While comparing the sizes of 141 packages built with -fpic vs -fPIC,
most packages are either equal or smaller in size. Only 9 of the
compared packages turned out slightly larger.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>