[ci] Include UEFI Secure Boot builds for RISC-V 64 and LoongArch64

[ci] Include UEFI Secure Boot builds for RISC-V 64 and LoongArch64

The usage pattern for UEFI Secure Boot on RISC-V 64 and LoongArch64 is
not yet well defined: there is no equivalent on those architectures
for the UEFI shim or the Microsoft signing submission infrastructure.

Include signed binaries for these architectures within the release
artifacts.  Users may choose to enrol the iPXE Secure Boot CA
certificate on their own systems in order to use these binaries with
UEFI Secure Boot enabled.

OEMs such as Loongson may choose to include the iPXE Secure Boot CA
certificate within their default enrolled certificate list, or to
issue a cross-signed version of the iPXE Secure Boot CA certificate
(which could then be included within the official iPXE binaries in
future releases).

Signed-off-by: Michael Brown <mcb30@ipxe.org>