Add an enum for the riscv64 <sys/ucontext.h> register constants.

That lets us have idempotent #defines to reduce namespace pollution.

Bug: http://b/279405445
Test: treehugger
Change-Id: I00312cc8911025696cf0eea2d70e3219ab361613

libc/include/sys/ucontext.h

@@ -318,12 +318,20 @@ typedef struct ucontext {
 
 #if defined(__USE_GNU)
 
-#define REG_PC 0
-#define REG_RA 1
-#define REG_SP 2
-#define REG_TP 4
-#define REG_S0 8
-#define REG_A0 10
+enum {
+  REG_PC = 0,
+#define REG_PC REG_PC
+  REG_RA = 1,
+#define REG_RA REG_RA
+  REG_SP = 2,
+#define REG_SP REG_SP
+  REG_TP = 4,
+#define REG_TP REG_TP
+  REG_S0 = 8,
+#define REG_S0 REG_S0
+  REG_A0 = 10,
+#define REG_A0 REG_A0
+};
 
 #endif // defined(__USE_GNU)