roms/edk2-funcs: Force softfloat ARM toolchain prefix on Debian

roms/edk2-funcs: Force softfloat ARM toolchain prefix on Debian

The Debian (based) distributions currently provides 2 ARM
toolchains, documented as [1]:

* The ARM EABI (armel) port targets a range of older 32-bit ARM
  devices, particularly those used in NAS hardware and a variety
  of *plug computers.
* The newer ARM hard-float (armhf) port supports newer, more
  powerful 32-bit devices using version 7 of the ARM architecture
  specification.

For various reasons documented in [2], the EDK2 project suggests
to use the softfloat toolchain (named 'armel' by Debian).

Force the softfloat cross toolchain prefix on Debian distributions.

[1] https://www.debian.org/ports/arm/#status
[2] https://github.com/tianocore/edk2/commit/41203b9a

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>