ext: Revert "base: Use system libelf instead of ext"

This reverts commit bbb32ca1ef,
which tells scons to use the system's libelf instead of
ext/libelf.

Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Change-Id: I3bb3e62f2ef0fbc72983c221d5570edb4b35d157
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33940
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -716,10 +716,6 @@ if not conf.CheckLibWithHeader('z', 'zlib.h', 'C++','zlibVersion();'):
           'and/or zlib.h header file.\n'
           'Please install zlib and try again.')
 
-if not conf.CheckLibWithHeader('elf', 'gelf.h', 'C++',
-                               'elf_version(EV_CURRENT);'):
-    error('Did not find ELF access library libelf')
-
 # If we have the protobuf compiler, also make sure we have the
 # development libraries. If the check passes, libprotobuf will be
 # automatically added to the LIBS environment variable. After