scons: allow passing arbitrary CCFLAGS and LDFLAGS from the CLI

scons: allow passing arbitrary CCFLAGS and LDFLAGS from the CLI

The flags may be passed as:

scons CCFLAGS_EXTRA='-Wno-error -pedantic' \
      LDFLAGS_EXTRA='-g -g' build/<arch>/gem5.opt

The initial motivation for this commit is to help disable warning that
have become errors while bisecting.

scons orders the flags by Append call order, and ideally these flags
should be added last to override the others, since the last GCC flags
take precedence. However I haven't found a simple way to put them at
the very end.

Change-Id: Ida24dfb9604d88b99f113392ab5e47d578ba7259
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19048
Reviewed-by: Juha Jäykkä <juha.jaykka@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>