target-arm: Fix warn about implicit conversion

target-arm: Fix warn about implicit conversion

Clang warns about an implicit conversion as follows:

/mnt/devops/code/qemu/target-arm/neon_helper.c:1075:1: warning: implicit conversion from 'int' to 'int8_t' (aka 'signed char') changes value from 128 to -128 [-Wconstant-conversion]
NEON_VOP_ENV(qrshl_s8, neon_s8, 4)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/mnt/devops/code/qemu/target-arm/neon_helper.c:116:83: note: expanded from macro 'NEON_VOP_ENV'
uint32_t HELPER(glue(neon_,name))(CPUARMState *env, uint32_t arg1, uint32_t arg2) \
                                                                                  ^
/mnt/devops/code/qemu/target-arm/neon_helper.c:106:5: note: expanded from macro '\
NEON_VOP_BODY'
    NEON_DO##n; \
    ^~~~~~~~~~
<scratch space>:21:1: note: expanded from here
NEON_DO4
^~~~~~~~
/mnt/devops/code/qemu/target-arm/neon_helper.c:93:5: note: expanded from macro 'NEON_DO4'
    NEON_FN(vdest.v1, vsrc1.v1, vsrc2.v1); \
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/mnt/devops/code/qemu/target-arm/neon_helper.c:1054:23: note: expanded from macro 'NEON_FN'
            dest = (1 << (sizeof(src1) * 8 - 1)); \
                 ~  ~~^~~~~~~~~~~~~~~~~~~~~~~~~

Fix it by casting to appropriate type.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>