GCC toolchain: Add {SYSROOT} substitution with support for whitespaces

Any instance of {SYROOT} string in config's CFLAGS and LDFLAGS is replaced by sysroot path obtained from "gcc -print-sysroot"

<?xml version='1.0' encoding='utf-8'?>
<BaseParams xmlns:xsd="http://www.w3.org/2001/XMLSchema" IEC_Channel="1" Name="py_ext_0"/>