tools: selftests: riscv: Fix compile warnings in mm tests

When building the mm tests with a riscv32 compiler, we see a range
of shift-count-overflow errors from shifting 1UL by more than 32 bits
in do_mmaps(). Since, the relevant code is only called from code that
is gated by `__riscv_xlen == 64`, we can just apply the same gating
to do_mmaps().
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20231123185821.2272504-6-christoph.muellner@vrull.euSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

tools/testing/selftests/riscv/mm/mmap_test.h

@@ -18,6 +18,8 @@ struct addresses {
 	int *on_56_addr;
 };
 
+// Only works on 64 bit
+#if __riscv_xlen == 64
 static inline void do_mmaps(struct addresses *mmap_addresses)
 {
 	/*
@@ -50,6 +52,7 @@ static inline void do_mmaps(struct addresses *mmap_addresses)
 	mmap_addresses->on_56_addr =
 		mmap(on_56_bits, 5 * sizeof(int), prot, flags, 0, 0);
 }
+#endif /* __riscv_xlen == 64 */
 
 static inline int memory_layout(void)
 {