target/arm: Drop unsupported_encoding() macro

The unsupported_encoding() macro logs a LOG_UNIMP message and then
generates code to raise the usual exception for an unallocated
encoding.  Back when we were still implementing the A64 decoder this
was helpful for flagging up when guest code was using something we
hadn't yet implemented.  Now we completely cover the A64 instruction
set it is barely used.  The only remaining uses are for five
instructions whose semantics are "UNDEF, unless being run under
external halting debug":
 * HLT (when not being used for semihosting)
 * DCPSR1, DCPS2, DCPS3
 * DRPS

QEMU doesn't implement external halting debug, so for us the UNDEF is
the architecturally correct behaviour (because it's not possible to
execute these instructions with halting debug enabled).  The
LOG_UNIMP doesn't serve a useful purpose; replace these uses of
unsupported_encoding() with unallocated_encoding(), and delete the
macro.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220509160443.3561604-1-peter.maydell@linaro.org