sys/boot: Unbreak efi build with WORLD_LDVER=ld.bfd.
Adjust 64-bit libstand.a location to use full path if available as it is
done for 32-bit loaders already.
The ld.bfd when invoked with -ffreestanding will not look for /usr/lib.
Match ldscript.x86_64 to sys/platform/pc64/conf/ldscript.x86_64.
Our ld.bfd does not support freebsd emulation target. Generic target is
enough to convert boot1.sym and loader.sym intermediates to PE32+.
While there, use something sane for section padding in ldscript.x86_64.