Added links to references

Author: vhscampos

libc/startup/baremetal/arm/start.cpp

@@ -134,12 +134,16 @@ namespace LIBC_NAMESPACE_DECL {
 #ifdef __ARM_FP
 // Enable FPU
 #if __ARM_ARCH_PROFILE == 'M'
+  // Based on
+  // https://developer.arm.com/documentation/dui0646/c/Cortex-M7-Peripherals/Floating-Point-Unit/Enabling-the-FPU
   // Set CPACR cp10 and cp11
   auto cpacr = (volatile uint32_t *const)0xE000ED88;
   *cpacr |= (0xF << 20);
   __dsb(0xF);
   __isb(0xF);
 #elif __ARM_ARCH_PROFILE == 'A' || __ARM_ARCH_PROFILE == 'R'
+  // Based on
+  // https://developer.arm.com/documentation/dui0472/m/Compiler-Coding-Practices/Enabling-NEON-and-FPU-for-bare-metal
   // Set CPACR cp10 and cp11
   uint32_t cpacr = __arm_rsr("p15:0:c1:c0:2");
   cpacr |= (0xF << 20);