scons: Include libthr on freebsd no matter what compiler you're using.

The compiler won't change where the thread library is implemented.

Change-Id: Ic6fbaae2ff9c4a91ce86b06d62049b9d9a0e3132
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40960
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: B.A. Zeeb <baz21@cam.ac.uk>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -305,15 +305,20 @@ if main['GCC'] or main['CLANG']:
     # As gcc and clang share many flags, do the common parts here
     main.Append(CCFLAGS=['-pipe'])
     main.Append(CCFLAGS=['-fno-strict-aliasing'])
+
     # Enable -Wall and -Wextra and then disable the few warnings that
     # we consistently violate
     main.Append(CCFLAGS=['-Wall', '-Wundef', '-Wextra',
                          '-Wno-sign-compare', '-Wno-unused-parameter'])
+
     # We always compile using C++14
     main.Append(CXXFLAGS=['-std=c++14'])
+
     if sys.platform.startswith('freebsd'):
         main.Append(CCFLAGS=['-I/usr/local/include'])
         main.Append(CXXFLAGS=['-I/usr/local/include'])
+        # On FreeBSD we need libthr.
+        main.Append(LIBS=['thr'])
 
     conf.CheckLinkFlag('-Wl,--as-needed')
     if GetOption('gold_linker'):
@@ -386,10 +391,6 @@ elif main['CLANG']:
         main.Append(CXXFLAGS=['-stdlib=libc++'])
         main.Append(LIBS=['c++'])
 
-    # On FreeBSD we need libthr.
-    if sys.platform.startswith('freebsd'):
-        main.Append(LIBS=['thr'])
-
 # Add sanitizers flags
 sanitizers=[]
 if GetOption('with_ubsan'):