Update ARM inline assembly constraints.

Inline assembly in uECC_vli_add is underspecified; the output
registers are written to before the input registers are fully
consumed. Updating the constraints to include earlyclobber `&`.

bug: b/278442070
Change-Id: I1f4c28aeca875c7cdcf1d0847d5dbebc7835b081
Reviewed-on: https://pigweed-review.googlesource.com/c/third_party/github/kmackay/micro-ecc/+/139830
Reviewed-by: Ali Zhang <alizhang@google.com>

asm_arm.inc

diff --git a/asm_arm.inc b/asm_arm.inc
index e3c8cb1..12e747f 100644
--- a/asm_arm.inc
+++ b/asm_arm.inc
@@ -20,19 +20,19 @@
 #endif
 
 #if (uECC_PLATFORM == uECC_arm_thumb)
-    #define REG_RW "+l"
-    #define REG_WRITE "=l"
+    #define REG_RW "+&l"
+    #define REG_WRITE "=&l"
 #else
-    #define REG_RW "+r"
-    #define REG_WRITE "=r"
+    #define REG_RW "+&r"
+    #define REG_WRITE "=&r"
 #endif
 
 #if (uECC_PLATFORM == uECC_arm_thumb || uECC_PLATFORM == uECC_arm_thumb2)
-    #define REG_RW_LO "+l"
-    #define REG_WRITE_LO "=l"
+    #define REG_RW_LO "+&l"
+    #define REG_WRITE_LO "=&l"
 #else
-    #define REG_RW_LO "+r"
-    #define REG_WRITE_LO "=r"
+    #define REG_RW_LO "+&r"
+    #define REG_WRITE_LO "=&r"
 #endif
 
 #if (uECC_PLATFORM == uECC_arm_thumb2)