build/gcc.full.build: fix gcc.{host,stage1}.mingw_w64 build.
patches/gcc.stage1.mingw_w64.local.patch: force libiberty/pex-unix.c when targeting mingw_w64.
subr/build.subr: create ${PREFIX}/man -> share/man symlink as w/ ${PREFIX_NATIVE}.
subr/rtl.subr: obtain target from ${PKG_TARGET} in set_build_dir().
vars/build.vars: adds ${PKG_TARGET} var; reenables gcc.stage1.mingw_w64.