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"

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<?xml version='1.0' encoding='utf-8'?>

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<BeremizRoot URI_location="LOCAL://">

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  <TargetType/>

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  <Libraries Enable_Python_Library="true"/>

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</BeremizRoot>