test: let the shim compile sources that touch the SFRs and libc

Two things kept every register-driven module out of the host harness. The
SFR keywords were unknown, so rtl837x_sfr.h did not compile at all, and once
it did, each translation unit would have defined its own copy of the
registers. They map to extern declarations now; a mock emits the storage
once by re-including the header with the keywords redefined.

The other was memset. rtl837x_common.h hides the firmware's own prototypes
under RTLP_HOST_TEST so host builds use the C library, but nothing declared
the library versions, and the modules do call them. The shim pulls in
string.h ahead of everything, where nothing can clash with it.

test/sdcc_shim.h

@@ -12,6 +12,7 @@
 #define SDCC_SHIM_H
 
 #include <stdbool.h>
+#include <stdint.h>
 
 /* Memory-space qualifiers → nothing (host has one flat address space). */
 #define __xdata
@@ -22,12 +23,27 @@
 /* bool, not unsigned char: SDCC's bit types normalize any assigned value
  * to 0/1, which _Bool reproduces exactly. */
 #define __bit         bool
-#define __sbit        bool
 
-/* NOTE: the firmware ships its own 8051 memcpy/memset/strlen/strcpy/sleep with
+/* Special-function registers are declared in rtl837x_sfr.h as
- * incompatible signatures (rtl837x_common.h). Host TUs must therefore build with
+ * `__sfr __at(0xNN) NAME;`. With __at erased those become plain declarations,
- * -fno-builtin (so gcc doesn't auto-declare the builtins) and must NOT include
+ * so they must carry extern or every translation unit including the header
- * <string.h>/<unistd.h>. Test code uses the t_* helpers in support.h instead. */
+ * defines its own copy. __sbit is the bit-in-an-SFR form and only appears
+ * there; the local bit variable keyword is __bit above. test/hw_mock.c
+ * undefines these four and re-includes the header once to emit the storage. */
+#define __sfr         extern volatile uint8_t
+#define __sfr16       extern volatile uint16_t
+#define __sfr32       extern volatile uint32_t
+#define __sbit        extern bool
+
+/* The firmware ships its own memcpy/memset/strlen/strcpy/sleep, and
+ * rtl837x_common.h hides those prototypes under RTLP_HOST_TEST so host builds
+ * take the C library versions. Something has to declare them, and firmware
+ * sources do call memset and memcpy; this header is force-included ahead of
+ * everything, so pulling <string.h> in here puts the libc versions in scope
+ * before any firmware header and nothing clashes. sleep() is not in
+ * <string.h> and is declared on its own rather than through <unistd.h>. */
+#include <string.h>
+extern unsigned int sleep(unsigned int seconds);
 
 /* Function attributes → nothing (no banking / no 8051 calling conventions). */
 #define __banked