libunwind: Remove UNW_ARM64_ECSP

Stop treating the trusted stack pointer as a register because it is
unnecessary and some architectures like RISC-V do not in fact store it
in a dedicated register.

Author: dpgao

libunwind/include/libunwind.h

@@ -679,10 +679,6 @@ enum {
   UNW_ARM64_CLR = 228,
   UNW_ARM64_C31 = 229,
   UNW_ARM64_CSP = 229,
-  // Use 240 for ECSP (executive stack pointer). ECSP is not a real DWARF
-  // register, but we need it to implement c18n-aware unwinding. We pick 240
-  // because it is far enough away from the range of reserved registers on Arm.
-  UNW_ARM64_ECSP = 240,
 };
 
 // 32-bit ARM registers. Numbers match DWARF for ARM spec #3.1 Table 1.

libunwind/src/Registers.hpp

@@ -2206,8 +2206,6 @@ inline const char *Registers_arm64::getRegisterName(int regNum) {
     return "clr";
   case UNW_ARM64_C31:
     return "csp";
-  case UNW_ARM64_ECSP:
-    return "ecsp";
   default:
     return "unknown register";
   }