proc: fix PIE proc-empty-vm, proc-pid-vm tests

vsyscall detection code uses direct call to the beginning of
the vsyscall page:

asm ("call %P0" :: "i" (0xffffffffff600000))

It generates "call rel32" instruction but it is not relocated if binary
is PIE, so binary segfaults into random userspace address and vsyscall
page status is detected incorrectly.

Do more direct:

asm ("call *%rax")

which doesn't do need any relocaltions.

Mark g_vsyscall as volatile for a good measure, I didn't find instruction
setting it to 0. Now the code is obviously correct:

xor eax, eax
mov rdi, rbp
mov rsi, rbp
mov DWORD PTR [rip+0x2d15], eax # g_vsyscall = 0
mov rax, 0xffffffffff600000
call rax
mov DWORD PTR [rip+0x2d02], 1 # g_vsyscall = 1
mov eax, DWORD PTR ds:0xffffffffff600000
mov DWORD PTR [rip+0x2cf1], 2 # g_vsyscall = 2
mov edi, [rip+0x2ceb] # exit(g_vsyscall)
call exit

Note: fixed proc-empty-vm test oopses 5.19.0-28-generic kernel
but this is separate story.

Link: https://lkml.kernel.org/r/Y7h2xvzKLg36DSq8@p183
Fixes: 5bc73bb3451b9 ("proc: test how it holds up with mapping'less process")
Signed-off-by: Alexey Dobriyan <[e-mail]>
Reported-by: Mirsad Goran Todorovac <[e-mail]>
Tested-by: Mirsad Goran Todorovac <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/proc/proc-empty-vm.c

···
 #undef NDEBUG
 #include <assert.h>
 #include <errno.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
···
  * 1: vsyscall VMA is --xp		vsyscall=xonly
  * 2: vsyscall VMA is r-xp		vsyscall=emulate
  */
-static int g_vsyscall;
+static volatile int g_vsyscall;
 static const char *g_proc_pid_maps_vsyscall;
 static const char *g_proc_pid_smaps_vsyscall;
 
···
 
 		g_vsyscall = 0;
 		/* gettimeofday(NULL, NULL); */
+		uint64_t rax = 0xffffffffff600000;
 		asm volatile (
-			"call %P0"
-			:
-			: "i" (0xffffffffff600000), "D" (NULL), "S" (NULL)
-			: "rax", "rcx", "r11"
+			"call *%[rax]"
+			: [rax] "+a" (rax)
+			: "D" (NULL), "S" (NULL)
+			: "rcx", "r11"
 		);
 
 		g_vsyscall = 1;

tools/testing/selftests/proc/proc-pid-vm.c

···
 
 		g_vsyscall = 0;
 		/* gettimeofday(NULL, NULL); */
+		uint64_t rax = 0xffffffffff600000;
 		asm volatile (
-			"call %P0"
-			:
-			: "i" (0xffffffffff600000), "D" (NULL), "S" (NULL)
-			: "rax", "rcx", "r11"
+			"call *%[rax]"
+			: [rax] "+a" (rax)
+			: "D" (NULL), "S" (NULL)
+			: "rcx", "r11"
 		);
 
 		g_vsyscall = 1;