Fix TestRegisterVariables.py on Windows

[amccarth]

Summary

32-bit Windows calling conventions, by default, don't pass arguments in registers, but this test expects at least one of them to be.

By adding attribute((regparm(x))) to the functions, we can get the compiler to enregister some of the parameters, and the test passes.

I don't think this should have a negative impact on other platforms, but I haven't tried.

[tfiala]

Seems reasonable. We can revert and fix if this breaks something.

lldb/trunk/packages/Python/lldbsuite/test/lang/c/register_variables/test.c

 #include <stdio.h>
 
 struct bar {
   int m1;
   int m2;
 };
 
-void f1(int a, struct bar *b) __attribute__ ((noinline));
+void f1(int a, struct bar *b) __attribute__((noinline)) __attribute__((regparm(2)));
 void f1(int a, struct bar *b)
 {
   b->m2 = b->m1 + a; // set breakpoint here
 }
 
-void f2(struct bar *b) __attribute__ ((noinline));
+void f2(struct bar *b) __attribute__((noinline)) __attribute__((regparm(1)));
 void f2(struct bar *b)
 {
   int c = b->m2;
   printf("%d\n", c); // set breakpoint here
 }
 
-float f3() __attribute__ ((noinline));
+float f3() __attribute__((noinline));
 float f3() {
   return 3.14f;
 }
 
 int main()
 {
   struct bar myBar = { 3, 4 };
   f1(2, &myBar);
   f2(&myBar);
 
   float f = f3();
   printf("%f\n", f); // set breakpoint here
   return 0;
 }