scons: Add --with-lto to enabled LTO; remove --no-lto

LTO is no longer enabled by default and can instead be enabled by
`--with-lto`.

Change-Id: I2eea7d447703491675c02730707cf9026cface5f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44887
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -1435,11 +1435,9 @@ if env['GCC']:
     # the optimization to the ldflags as LTO defers the optimization
     # to link time
     for target in ['opt', 'fast', 'prof', 'perf']:
-        ccflags[target] += ['-O3']
-        ldflags[target] += ['-O3']
+        ccflags[target] += ['-O3'] + env['LTO_CCFLAGS']
+        ldflags[target] += ['-O3'] + env['LTO_LDFLAGS']
 
-    ccflags['fast'] += env['LTO_CCFLAGS']
-    ldflags['fast'] += env['LTO_LDFLAGS']
 elif env['CLANG']:
     ccflags['debug'] += ['-g', '-O0']
     # opt, fast, prof and perf all share the same cc flags