riscv64: add PROMOTE_RET for narrow return ABI compliance

Other mature backends (i386, x86_64, arm64) all define PROMOTE_RET,
which forces explicit sign/zero extension of narrow return types at the
caller side.  Without it, riscv64 relied on the assumption that RV64
registers are always sign-extended, which may not hold when interfacing
with non-TCC compilers.

Verified on Spacemit X100 (riscv64): self-compilation is self-consistent
across 3 layers.  test3 reference mismatch is expected — the test
intentionally invokes UB via type-punned function pointers (csf macro).

riscv64-gen.c

@@ -30,6 +30,10 @@
 
 #define CHAR_IS_UNSIGNED
 
+/* define if return values need to be extended explicitely
+   at caller side (for interfacing with non-TCC compilers) */
+#define PROMOTE_RET
+
 #else
 #define USING_GLOBALS
 #include "tcc.h"

tests/tests2/138_narrow_return_promotion.c

@@ -0,0 +1,52 @@
+#include <stdio.h>
+
+/* PROMOTE_RET test: verify char/short return values are properly
+   zero/sign-extended to 64-bit per the RISC-V integer calling convention.
+   Without PROMOTE_RET, upper bits of the return register may contain
+   garbage, causing incorrect results when assigned to a wider type.  */
+
+unsigned char get_uc(void) { return 0x80; }
+signed char get_sc(void) { return 0x80; }
+unsigned short get_us(void) { return 0x8000; }
+signed short get_ss(void) { return 0x8000; }
+
+/* Prevent inlining to force ABI-compliant calling.  */
+unsigned char (* volatile fp_uc)(void) = get_uc;
+signed char (* volatile fp_sc)(void) = get_sc;
+unsigned short (* volatile fp_us)(void) = get_us;
+signed short (* volatile fp_ss)(void) = get_ss;
+
+int main(void)
+{
+    int ok = 1;
+    unsigned long long uc = fp_uc();
+    signed long long sc = fp_sc();
+    unsigned long long us = fp_us();
+    signed long long ss = fp_ss();
+
+    printf("uc=%llx sc=%llx us=%llx ss=%llx\n",
+           (unsigned long long)uc,
+           (unsigned long long)sc,
+           (unsigned long long)us,
+           (unsigned long long)ss);
+
+    if (uc != 0x80) {
+        printf("FAIL: uc not zero-extended\n");
+        ok = 0;
+    }
+    if (sc != 0xffffffffffffff80LL) {
+        printf("FAIL: sc not sign-extended\n");
+        ok = 0;
+    }
+    if (us != 0x8000) {
+        printf("FAIL: us not zero-extended\n");
+        ok = 0;
+    }
+    if (ss != 0xffffffffffff8000LL) {
+        printf("FAIL: ss not sign-extended\n");
+        ok = 0;
+    }
+
+    printf("%s\n", ok ? "PASS" : "FAIL");
+    return ok ? 0 : 1;
+}

tests/tests2/138_narrow_return_promotion.expect

@@ -0,0 +1,2 @@
+uc=80 sc=ffffffffffffff80 us=8000 ss=ffffffffffff8000
+PASS