[Sanitizer][RISCV] Fix redefinition of REG_SP

[luismarques]

Summary

The include header sys/ucontext.h already defines REG_SP as 2, causing redefinition warnings during compilation. This patch fixes that issue.
(We also can't just use the numerical definition provided by the header, as REG_SP is used in this file this refers to a struct field.)

[EccoTheDolphin]

LGTM

compiler-rt/lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cpp

 #elif defined(__aarch64__)
 typedef struct user_pt_regs regs_struct;
 #define REG_SP sp
 static constexpr uptr kExtraRegs[] = {0};
 #define ARCH_IOVEC_FOR_GETREGSET
 
 #elif SANITIZER_RISCV64
 typedef struct user_regs_struct regs_struct;
+// sys/ucontext.h already defines REG_SP as 2. Undefine it first.
+#undef REG_SP
 #define REG_SP sp
 static constexpr uptr kExtraRegs[] = {0};
 #define ARCH_IOVEC_FOR_GETREGSET
 
 #elif defined(__s390__)
 typedef _user_regs_struct regs_struct;
 #define REG_SP gprs[15]
 static constexpr uptr kExtraRegs[] = {0};