Remove -fPIC on x86_64

The -fPIC breaks the purgatory of kexec (= the code that passes command-line
arguments to the kernel): kexec doesn't know how to handle the GOT and PLT
relocation entries.

There is also no reason at all to pass -fPIC.  Shared libraries that require
it will add it to their local Makefiles, and normal executables have no
business with -fPIC (plus it adds overhead...).

The -fPIC was added by Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
in commit 8027784c.  That commit adds sysroot parameters to CFLAGS.
There is no explanation why -fPIC is also added for x86_64.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>