Add ppc64le architecture support for ptrace process tracing

On ppc64le (PowerPC 64-bit little-endian), the register struct is
pt_regs (from asm/ptrace.h) rather than user_regs_struct, and the
instruction pointer register is 'nip'. Without this, the code falls
through to the x86 #else branch which fails to compile because
user_regs_struct does not exist on ppc64le.

Define the appropriate macros:
- Map user_regs_struct to pt_regs
- Set INST_PTR_REG to nip
- Set BREAK_INST to 0x7fe00008 (ppc64 trap instruction)
- Set BREAK_INST_BYTES_SIZE to 4
- Set BREAK_INST_INST_PTR_ADJUST to 0

renderdoc/os/posix/linux/linux_process.cpp

@@ -243,6 +243,16 @@ static uint64_t get_nanotime()
 #define BREAK_INST_BYTES_SIZE 4
 #define BREAK_INST_INST_PTR_ADJUST 4
 
+#elif defined(__powerpc64__)
+
+#define user_regs_struct pt_regs
+#define INST_PTR_REG nip
+
+// trap instruction on ppc64
+#define BREAK_INST 0x7fe00008ULL
+#define BREAK_INST_BYTES_SIZE 4
+#define BREAK_INST_INST_PTR_ADJUST 0
+
 #else
 
 #define BREAK_INST 0xccULL