[build] Disable position-independent code for ARM64 EFI builds

[build] Disable position-independent code for ARM64 EFI builds

Some versions of gcc (observed with the cross-compiling gcc 9.3.0 in
Ubuntu 20.04) default to enabling -fPIE.  Experimentation shows that
this results in the emission of R_AARCH64_ADR_GOT_PAGE relocation
records for __stack_chk_guard.  These relocation types are not
supported by elf2efi.c.

Fix by explicitly disabling position-independent code for ARM64 EFI
builds.

Debugged-by: Antony Messerli <antony@mes.ser.li>
Signed-off-by: Michael Brown <mcb30@ipxe.org>