target-sh4: Split the LDST macro into 2 sub-macros

target-sh4: Split the LDST macro into 2 sub-macros

The LDST macro is used to generate ldc and stc instructions that work with a
specific register. However, the SGR register only supports stc up to SH4A,
which supports both stc and ldc. This patch creates two sub-macros named LD
and ST that handle generating ldc and stc instructions separately, and
redeclares LDST to use these sub-macro.

Signed-off-by: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>