arm: elf-fdpic.h: avoid void pointer subtraction

elf-fdpic.h is included by link.h. When a C++ program includes <link.h>,
we get the following build failure:

<...>/usr/include/bits/elf-fdpic.h: In function ‘void* __reloc_pointer(void*, const elf32_fdpic_loadmap*)’:
<...>/usr/include/bits/elf-fdpic.h:94:54: error: invalid use of ‘void’
   94 |       unsigned long offset = p - (void*)map->segs[c].p_vaddr;
      |                                                      ^~~~~~~

void pointer addition and subtraction is not allowed in C++ as it has
undetermined size, however in C with language extension it is possible
because sizeof void is treated as one byte.

This patch was previously applied to Blackfin, FR-V and C6x, but not
ARM.

Signed-off-by: Ben Wolsieffer <ben.wolsieffer@hefring.com>

libc/sysdeps/linux/arm/bits/elf-fdpic.h

@@ -91,7 +91,7 @@ __reloc_pointer (void *p,
       /* This should be computed as part of the pointer comparison
 	 above, but we want to use the carry in the comparison, so we
 	 can't convert it to an integer type beforehand.  */
-      unsigned long offset = p - (void*)map->segs[c].p_vaddr;
+      unsigned long offset = (char*)p - (char*)map->segs[c].p_vaddr;
       /* We only check for one-past-the-end for the last segment,
 	 assumed to be the data segment, because other cases are
 	 ambiguous in the absence of padding between segments, and