cmake: For gcc8 use linker to find paths to libc, libm, libstdc++, etc

Removes the need to know/guess the paths to these libraries. Once we are gcc 8 only, we
can remove -nostdlib and no additional arguments are needed for system libraries.

The catch is: any time IDF overrides a symbol in the toolchain sysroot, we need
an undefined linker marker to make sure this symbol is seen by linker.

components/bootloader/subproject/main/bootloader_start.c

@@ -111,3 +111,9 @@ static int selected_boot_partition(const bootloader_state_t *bs)
     }
     return boot_index;
 }
+
+// Return global reent struct if any newlib functions are linked to bootloader
+struct _reent* __getreent() {
+    return _GLOBAL_REENT;
+}
+