mescc: Tinycc support: Char and short sign extend.
* module/mes/as-i386.mes (i386:accu*n->local, i386:byte-accu->local+n,
i386:word-accu->local+n): Remove.
(i386:byte-accu, i386:signed-byte-accu, i386:word-accu,
i386:signed-word-accu): New function.
* module/mes/as-i386.scm: Export them.
* module/language/c99/compiler.mes (accu->local+n-text): Simplify.
(mem->accu, convert-accu): New function.
(expr->accu): Use them.
* stage0/x86.M1 (mov____%al,0x32(%ebp), mov____%al,0x8(%ebp),
mov____%ax,0x32(%ebp), mov____%ax,0x8(%ebp), movzbl_0x32(%ebp),%eax,
movzbl_0x8(%ebp),%eax, movzwl_0x32(%ebp),%eax,
movzwl_0x8(%ebp),%eax): Deprecate.
* scaffold/tests/7r-sign-extend.c: Test it.
* build-aux/check-mescc.sh (tests): Run it.
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