fix GCC sysroot for older versions

# Returns the location of the libc.a file for the given compiler + flags

define toolchain_find_libc_a

$$(readlink -f $$(LANG=C $(1) -print-file-name=libc.a))

$$(readlink -f $$(LANG=C $(1) --sysroot=$(TOOLCHAIN_EXTERNAL_INSTALL_DIR) -print-file-name=libc.a))

endef

# Returns the sysroot location for the given compiler + flags

define toolchain_find_sysroot

$$(echo -n $(call toolchain_find_libc_a,$(1)) | sed -r -e 's:(usr/)?lib(32|64)?/([^/]*/)?libc\.a::')

$$(echo -n $(call toolchain_find_libc_a,$(1)) | sed -r -e 's:/(usr/)?lib(32|64)?([^/]*)?/([^/]*/)?libc\.a:/:')

endef

# Returns the lib subdirectory for the given compiler + flags (i.e