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
Test: verified that this fixes #593 in the NDK
Bug: https://github.com/android-ndk/ndk/issues/593
Change-Id: Ib35a4b6c82920666ae8a45aa0a2d43e458699b51