libbpf: Add 6th argument support for x86-64 in bpf_tracing.h

Add r9 as register containing 6th argument on x86-64 architecture, as
per its ABI. Add also a link to a page describing ABI for easier future
reference.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230120200914.3008030-3-andrii@kernel.org

tools/lib/bpf/bpf_tracing.h

@@ -78,6 +78,10 @@
 
 #if defined(bpf_target_x86)
 
+/*
+ * https://en.wikipedia.org/wiki/X86_calling_conventions#System_V_AMD64_ABI
+ */
+
 #if defined(__KERNEL__) || defined(__VMLINUX_H__)
 
 #define __PT_PARM1_REG di
@@ -85,6 +89,7 @@
 #define __PT_PARM3_REG dx
 #define __PT_PARM4_REG cx
 #define __PT_PARM5_REG r8
+#define __PT_PARM6_REG r9
 #define __PT_RET_REG sp
 #define __PT_FP_REG bp
 #define __PT_RC_REG ax
@@ -115,6 +120,7 @@
 #define __PT_PARM3_REG rdx
 #define __PT_PARM4_REG rcx
 #define __PT_PARM5_REG r8
+#define __PT_PARM6_REG r9
 #define __PT_RET_REG rsp
 #define __PT_FP_REG rbp
 #define __PT_RC_REG rax