Try to make the boot loader boot entries less x86_64-specific

Do not hardcode "x86_64" in boot entries

There is nothing particularly architecture-specific in the boot entry files. Use the %ARCH% custom template identifier (which gets replaced with $arch from profiledef.sh) to make the profiles easier to port to other architectures.

Related to #193 (closed)

Remove the architecture from boot entry file names

Since the contents are not architecture-specific, then the files names should not be architecture-specific either.

For the systemd-boot entry for Memtest86+, name it after the UEFI architecture it supports.

configs/releng/efiboot/loader/entries/03-archiso-memtest86+x64.conf: show only on x64 UEFI

The Memtest86+ x64 EFI binary only works in x64 UEFI. There is no point in showing a non-working boot entry in systemd-boot on IA32 UEFI.

configs/*/grub/: do not show the UEFI architecture

Except for x86_64 systems with IA32 UEFI, in all other cases, the UEFI is native to the CPU architecture. If needed, this can be checked from a booted system, so there is no need to show it in the boot loader.

configs/*/grub/: add entries for the UEFI shell on other architectures

Detect all possible UEFI-architecture-specific file names for the UEFI shell. Only the one matching the used UEFI architecture will be shown.

Unlike boot managers such as rEFInd or systemd-boot, GRUB does not automatically create boot entries at runtime for the matching UEFI shell.

Related to #193 (closed)