[libc][picolibc] improve gcc picolibc support

components/libc/compilers/picolibc/SConscript

@@ -1,10 +1,16 @@
 import os
 from building import *
-from llvm_arm import *
 Import('rtconfig')
 
 group = []
 
+if rtconfig.PLATFORM == 'gcc':
+  from gcc import *
+elif rtconfig.PLATFORM == 'llvm-arm':
+  from llvm_arm import *
+else:
+  Return('group')
+
 picolibc_version = GetPicoLibcVersion(rtconfig)
 
 if picolibc_version and not GetDepend('RT_USING_EXTERNAL_LIBC'):