[lldb][AArch64] Document how to control the SVE/SSVE tests

A lot of these use defines that I made up for this purpose,
which is not obvious at first glance. Document that at the top
of each file.

Author: DavidSpickett

lldb/test/API/commands/register/register/aarch64_dynamic_regset/main.c

@@ -1,5 +1,9 @@
 #include <sys/auxv.h>
 
+// If this program receives 0 arguments, it will use non-streaming SVE
+// registers. If the number of arguments is >= 1, it will use streaming SVE
+// registers.
+
 #ifndef HWCAP2_SME
 #define HWCAP2_SME (1 << 23)
 #endif

lldb/test/API/commands/register/register/aarch64_sve_registers/rw_access_dynamic_resize/main.c

@@ -2,6 +2,9 @@
 #include <stdbool.h>
 #include <sys/prctl.h>
 
+// If USE_SSVE is defined, this program will use streaming mode SVE registers
+// instead of non-streaming mode SVE registers.
+
 #ifndef PR_SME_SET_VL
 #define PR_SME_SET_VL 63
 #endif

lldb/test/API/commands/register/register/aarch64_sve_registers/rw_access_static_config/main.c

@@ -1,6 +1,10 @@
 #include <stdbool.h>
 #include <sys/prctl.h>
 
+// If START_SSVE is defined, this program will start in streaming SVE mode
+// (it will later enter and exit streaming mode a few times). Otherwise, it
+// will start in non-streaming SVE mode.
+
 #ifndef PR_SME_SET_VL
 #define PR_SME_SET_VL 63
 #endif

lldb/test/API/commands/register/register/aarch64_sve_simd_registers/main.c

@@ -1,6 +1,16 @@
 #include <stdint.h>
 #include <sys/prctl.h>
 
+// If SSVE is defined, this program will start in streaming SVE mode. Otherwise,
+// if SVE is defined, it will start in non-streaming mode and activate the SVE
+// registers by writing to one of them. If neither SSVE or SVE are defined,
+// the program will start in non-streaming mode, with the SVE registers
+// inactive.
+//
+// For most programs the difference between inactive non-streaming SVE and
+// active is transparent. For lldb, there are some differences in how we use
+// ptrace in either scenario.
+
 // base is added to each value. If base = 2, then v0 = 2, v1 = 3, etc.
 void write_simd_regs(unsigned base) {
 #define WRITE_SIMD(NUM)                                                        \