gcc10 => 10.2.1, with fixed compilation (#4799)

* gcc10 which compiles with gcc 10.

* moar ruby

* FileUtils.ln_sf ->  FileUtils.ln_s

* sync to gcc_10_master, add -fpic

* add necessary "#{CREW_DEST_PREFIX}/bin/#{gcc_arch}-bin links

* bump to e73caa0fc12e4b33563d68ee58b8af882b2341e6

* sync with upstream

* bump to 67cbe5c8e68299442552f18139e0689646f9729d

* built with -flto

* update to 0f64123bde80a37c8d9aced69405e71848a23b9

* Use CREW_{,DEST_}MAN_PREFIX constants