Fix GetLoadingAddress() to work for Native Client executables

NaCl executables have ELF program headers that look like this (for the
original NaCl x86 GCC toolchain):

Program Header:
    LOAD off    0x00010000 vaddr 0x00020000 paddr 0x00020000 align 2**16
         filesz 0x00017ce0 memsz 0x00017ce0 flags r-x
    LOAD off    0x00030000 vaddr 0x10020000 paddr 0x10020000 align 2**16
         filesz 0x00001c98 memsz 0x00001c98 flags r--
    LOAD off    0x00040000 vaddr 0x10030000 paddr 0x10030000 align 2**16
         filesz 0x000025ec memsz 0x00002b88 flags rw-

or this (for the newer NaCl ARM GCC toolchain):

Program Header:
    LOAD off    0x00010000 vaddr 0x00020000 paddr 0x00020000 align 2**16
         filesz 0x000193b0 memsz 0x000193b0 flags r-x
    LOAD off    0x00000000 vaddr 0x10020000 paddr 0x10020000 align 2**16
         filesz 0x00000978 memsz 0x00000978 flags r--
    LOAD off    0x00001000 vaddr 0x10031000 paddr 0x10031000 align 2**16
         filesz 0x00000abc memsz 0x00000fac flags rw-

Fix GetLoadingAddress() to return the start address of the first
segment, 0x20000, in these cases.  Looking at p_offset for this isn't
correct, and the first segment doesn't have p_offset == 0 here because
NaCl can't map the ELF file headers as part of the first segment
(which is for validatable code only).

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3424
TEST= check addresses in output of "dump_syms" when run on NaCl nexe

Patch by Mark Seaborn <mseaborn@chromium.org>
Review URL: https://breakpad.appspot.com/588002/


git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1170 4c0a9323-5329-0410-9bdc-e9ce6186880e