Makefile: explicitly disable PIE

The various available RISC-V toolchains differ in their default
configuration regarding PIE, e.g. the buildroot RISC-V toolchain
has PIE disabled by default while the Debian toolchain has it
enabled by default.

OpenSBI currently doesn't support being built with PIE enabled,
therefore disable it explicitly by passing "-fno-pie -no-pie" in
CFLAGS.

Signed-off-by: Karsten Merker <merker@debian.org>
Acked-by: Anup Patel <anup.patel@wdc.com>