scons: Add '-Wl,--as-needed' to default LINKFLAGS

scons: Add '-Wl,--as-needed' to default LINKFLAGS

In current build flow, EXTRAS flag is evaluated before building gem5
tools and binaries. Such that, unneeded libraries may be linked into
gem5 binaries. Adding '-Wl,--as-needed' can fix this problem also
shrinks binaries.

Change-Id: Ifb001786a66b0dd9b29865e39a5740313002f250
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24003
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>