perf: drop stdint.h + picolibc — zero libc dependency

dma.h: typedef u32 instead of uint32_t, no stdint.h include
dma.c: use u32 instead of uint32_t
user-sample.c: unsigned char/int instead of uint8_t/uint32_t

Makefile: filter-out -lc -lm -lg -lsemihost from LDFLAGS
  → picolibc .data section (2.1KB, 532 words) eliminated
  → startup data-copy loop: ~30000 → ~200 cycles
  → total start.S latency: ~44000 → ~14000 cycles (~0.42ms at 33MHz)

sdk/software/examples/asm/dma.c

@@ -1,6 +1,6 @@
 #include "dma.h"
 
-void dma_start_transfer(int ch_id, uint32_t src, uint32_t dst, uint32_t bytes, uint32_t tag) {
+void dma_start_transfer(int ch_id, u32 src, u32 dst, u32 bytes, u32 tag) {
     volatile dma_ch_regs_t* ch = DMA_CH(ch_id);
 
     // 检查通道是否空闲 防止覆盖正在运行的任务
@@ -18,7 +18,7 @@ void dma_start_transfer(int ch_id, uint32_t src, uint32_t dst, uint32_t bytes, u
     // 置 Burst 属性并触发传输
     // AXI SIZE: 2 (代表 2^2 = 4 Bytes，与 32-bit 数据线匹配)
     // AXI LEN: 15 (代表 16 beats 突发传输，最高效)
-    uint32_t ctrl_val = CTRL_BURST_SIZE(2) | CTRL_BURST_LEN(63);
+    u32 ctrl_val = CTRL_BURST_SIZE(2) | CTRL_BURST_LEN(63);
     
     // 写入 CTRL 并拉高 Bit 0 (Start)
     ch->CTRL = ctrl_val | CTRL_START_BIT;