target/mips: Move sysemu TCG-specific code to tcg/sysemu/ subfolder

Move cp0_helper.c and mips-semi.c to the new tcg/sysemu/ folder,
adapting the Meson machinery.

Move the opcode definitions to tcg/sysemu_helper.h.inc.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210428170410.479308-20-f4bug@amsat.org>
diff --git a/target/mips/tcg/meson.build b/target/mips/tcg/meson.build
index b74fa04..2cffc5a 100644
--- a/target/mips/tcg/meson.build
+++ b/target/mips/tcg/meson.build
@@ -1,3 +1,6 @@
 if have_user
   subdir('user')
 endif
+if have_system
+  subdir('sysemu')
+endif