ARM: Handle signed values in r-byte-mem-add, r-word-mem-add.
* lib/arm-mes/arm.M1 (ldrb___%r0,(%r1)): Delete macro.
(ldrh___%r0,(%r0)): Delete macro.
(ldrsh__%r0,(%r0)): New macro.
(ldrh___%r0,(%r1)): Delete macro.
(ldrsh__%r0,(%r1)): New macro.
(ldrh___%r1,(%r1)): Delete macro.
(ldrsh__%r1,(%r1)): New macro.
(ldrh___%r2,(%r2)): Delete macro.
(ldrsh__%r2,(%r2)): New macro.
(ldrh___%r3,(%r3)): Delete macro.
(ldrsh__%r3,(%r3)): New macro.
(ldrsb__%r0,(%r1)): New macro.
* module/mescc/armv4/as.scm (armv4:r-byte-mem-add): Use ldrsb.
(armv4:r-word-mem-add): Use ldrsh, add____$i32,(%r0).
(armv4:word-mem->r): Use ldrsh.
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