[linux] Avoid building Linux-specific prefix on non-Linux builds

Reported-by: David Yeske <dyeske@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

diff --git a/src/arch/i386/prefix/linuxprefix.S b/src/arch/i386/core/linux/linuxprefix.S
similarity index 100%
rename from src/arch/i386/prefix/linuxprefix.S
rename to src/arch/i386/core/linux/linuxprefix.S

diff --git a/src/arch/x86_64/prefix/linuxprefix.S b/src/arch/x86_64/core/linux/linuxprefix.S
similarity index 100%
rename from src/arch/x86_64/prefix/linuxprefix.S
rename to src/arch/x86_64/core/linux/linuxprefix.S