[OpenMP] [Flang] Fix Issue llvm#76121

Root cause: Segmentation fault is caused by null pointer dereference inside the __kmpc_fork_call_if function at https://github.com/llvm/llvm-project/blob/main/openmp/runtime/src/z_Linux_asm.S#L1186 . __kmpc_fork_call_if is missing case to handle argc=0 .

Fix: Added a check inside the __kmp_invoke_microtask function to handle the case when argc is 0.

openmp/runtime/src/z_Linux_asm.S

@@ -1150,6 +1150,9 @@ KMP_LABEL(kmp_invoke_pass_parms):	// put 1st - 6th parms to pkfn in registers.
 	movq	%rdi, %rbx	// pkfn -> %rbx
 	leaq	__gtid(%rbp), %rdi // &gtid -> %rdi (store 1st parm to pkfn)
 	leaq	__tid(%rbp), %rsi  // &tid -> %rsi (store 2nd parm to pkfn)
+	// Check if argc is 0
+	cmpq $0, %rax
+	je KMP_LABEL(kmp_no_args) // Jump ahead
 
 	movq	%r8, %r11	// p_argv -> %r11
 
@@ -1195,6 +1198,7 @@ KMP_LABEL(kmp_1_exit):
 	cmovnsq	(%r11), %rdx	// p_argv[0] -> %rdx (store 3rd parm to pkfn)
 #endif // KMP_MIC
 
+	KMP_LABEL(kmp_no_args):
 	call	*%rbx		// call (*pkfn)();
 	movq	$1, %rax	// move 1 into return register;