Fix __gnu_Unwind_Find_exidx for static executables.

__exidx_start and __exidx_end are not actual values to be read, just
symbols placed before and after .ARM.exidx.

The default linker script for ARM static executables includes these
lines:

    __exidx_start = .;
   .ARM.exidx   : { *(.ARM.exidx*) }
    __exidx_end = .;

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0474c/BABEHEDA.html

Test: used __gnu_Unwind_Find_exidx in a static executable and showed
      non-bogus values
Bug: android/ndk#593
Change-Id: Ib35a4b6c82920666ae8a45aa0a2d43e458699b51

libc/arch-arm/bionic/exidx_static.c

@@ -41,6 +41,6 @@ extern unsigned __exidx_start;
 _Unwind_Ptr __gnu_Unwind_Find_exidx(_Unwind_Ptr pc __attribute__((unused)), 
                                     int *pcount)
 {
-	*pcount = (__exidx_end-__exidx_start)/8;
-	return (_Unwind_Ptr)__exidx_start;
+	*pcount = (&__exidx_end-&__exidx_start)/8;
+	return (_Unwind_Ptr)&__exidx_start;
 }