add support for manually ignore functions

This commit adds the KPATCH_IGNORE_FUNC() macro for ignoring functions
that may change as a side effect of a change in another function.  The
WARN class of macros, for example, embed the line number in an
instruction, which will cause the function to be detected as changed
when, in fact, there has been no functional change.

Partially addresses #316 

Test patches:

To see the problem
```
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index bf233b2..9bf17c6 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2322,6 +2322,7 @@ static unsigned long pvm_determine_end(struct vmap_area **pnext,
        const unsigned long vmalloc_end = VMALLOC_END & ~(align - 1);
        unsigned long addr;
 
+       pr_info("here!\n");
        if (*pnext)
                addr = min((*pnext)->va_start & ~(align - 1), vmalloc_end);
        else
```

Output:
```
vmalloc.o: changed function: pvm_determine_end
vmalloc.o: changed function: pcpu_get_vm_areas
```

To see how the macro fixes the problem
```
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index bf233b2..3876758 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2322,6 +2322,7 @@ static unsigned long pvm_determine_end(struct vmap_area **pnext,
 	const unsigned long vmalloc_end = VMALLOC_END & ~(align - 1);
 	unsigned long addr;
 
+	pr_info("here!\n");
 	if (*pnext)
 		addr = min((*pnext)->va_start & ~(align - 1), vmalloc_end);
 	else
@@ -2512,6 +2513,8 @@ err_free2:
 	kfree(vms);
 	return NULL;
 }
+#include "kpatch-macros.h"
+KPATCH_IGNORE_FUNC(pcpu_get_vm_areas);
 
 /**
  * pcpu_free_vm_areas - free vmalloc areas for percpu allocator
```

Output:
```
vmalloc.o: ignoring function pcpu_get_vm_areas
vmalloc.o: changed function: pvm_determine_end
```
Maybe name the section `.kpatch.ignore.funcs` since we'll presumably also be adding a `.kpatch.ignore.sections` section?