selftests/size: Fix SIGSEGV on 32-bit x86 due to uninitialized vDSO pointer

Commit 6b64a650f0b2 ("kselftest: Minimise dependency of get_size on C library interfaces") replaced direct C library calls with syscall() to avoid accessing uninitialized TLS on x86_64 when built with -nostartfiles.

However, on 32-bit x86 (i386), the C library's syscall() wrapper itself relies on CRT startup initialization to dispatch system calls via the vDSO entry point:
- In glibc (libc.a), syscall() executes "call *%gs:0x10", reading the __kernel_vsyscall pointer from the Thread Control Block (TCB).
- In Android Bionic (libc.a), syscall() calls __kernel_syscall(), which returns the global __libc_sysinfo pointer in .bss.

Because get_size is compiled with -static -ffreestanding -nostartfiles and defines its own _start, CRT and TLS initialization never run. As a result, calling libc's syscall() on 32-bit x86 dereferences an uninitialized %gs selector or jumps to NULL (0x0), crashing immediately with SIGSEGV.

Provide a local syscall() implementation on __i386__ that invokes "int $0x80" directly, avoiding the C library's syscall() wrapper and its CRT initialization dependency.

---
 tools/testing/selftests/size/get_size.c | 29 +++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

index 2980b1a63366..352c90d5c770 100644
@@ -32,6 +32,35 @@
 
 #define STDOUT_FILENO 1
 
+#ifdef __i386__
+#include <stdarg.h>
+
+/*
+ * On 32-bit x86, libc's syscall() wrapper invokes the vDSO entry point
+ * stored in TLS (%gs:0x10 in glibc) or a global variable (__libc_sysinfo
+ * in Bionic) initialized by CRT startup routines. Because this binary is
+ * built with -nostartfiles, those pointers remain uninitialized and
+ * calling libc's syscall() crashes with SIGSEGV. Use int $0x80 directly.
+ */
+long syscall(long num, ...)
+{
+	va_list ap;
+	long a1, a2, a3, ret;
+
+	va_start(ap, num);
+	a1 = va_arg(ap, long);
+	a2 = va_arg(ap, long);
+	a3 = va_arg(ap, long);
+	va_end(ap);
+
+	__asm__ volatile("int $0x80"
+		: "=a"(ret)
+		: "a"(num), "b"(a1), "c"(a2), "d"(a3)
+		: "memory", "cc");
+	return ret;
+}
+#endif
+
 static int print(const char *s)
 {
 	size_t len = 0;

2.56.0.rc1.315.gc6ed9934b7-goog