target/arm/tcg: Don't build AArch64 decodetree files for qemu-system-arm

Currently we list all the Arm decodetree files together and add them
unconditionally to arm_ss.  This means we build them for both
qemu-system-aarch64 and qemu-system-arm.  However, some of them are
AArch64-specific, so there is no need to build them for
qemu-system-arm.  (Meson is smart enough to notice that the generated
.c.inc file is not used by any objects that go into qemu-system-arm,
so we only unnecessarily run decodetree, not anything more
heavyweight like a recompile or relink, but it's still unnecessary
work.)

Split gen into gen_a32 and gen_a64, and only add gen_a64 for
TARGET_AARCH64 compiles.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20230718104628.1137734-1-peter.maydell@linaro.org

target/arm/tcg/meson.build

@@ -1,7 +1,11 @@
-gen = [
+gen_a64 = [
+  decodetree.process('a64.decode', extra_args: ['--static-decode=disas_a64']),
   decodetree.process('sve.decode', extra_args: '--decode=disas_sve'),
   decodetree.process('sme.decode', extra_args: '--decode=disas_sme'),
   decodetree.process('sme-fa64.decode', extra_args: '--static-decode=disas_sme_fa64'),
+]
+
+gen_a32 = [
   decodetree.process('neon-shared.decode', extra_args: '--decode=disas_neon_shared'),
   decodetree.process('neon-dp.decode', extra_args: '--decode=disas_neon_dp'),
   decodetree.process('neon-ls.decode', extra_args: '--decode=disas_neon_ls'),
@@ -13,10 +17,10 @@ gen = [
   decodetree.process('a32-uncond.decode', extra_args: '--static-decode=disas_a32_uncond'),
   decodetree.process('t32.decode', extra_args: '--static-decode=disas_t32'),
   decodetree.process('t16.decode', extra_args: ['-w', '16', '--static-decode=disas_t16']),
-  decodetree.process('a64.decode', extra_args: ['--static-decode=disas_a64']),
 ]
 
-arm_ss.add(gen)
+arm_ss.add(gen_a32)
+arm_ss.add(when: 'TARGET_AARCH64', if_true: gen_a64)
 
 arm_ss.add(files(
   'cpu32.c',