sys/dev/md: Drop unncessary __GLOBL(mfs_root)

LLVM12 complains if you change the symbol binding:
error: mfs_root_end changed binding to STB_WEAK [-Werror,-Winline-asm]
error: mfs_root changed binding to STB_WEAK [-Werror,-Winline-asm]

(cherry picked from commit 69e18c9b7b12e7fd97a740d8748d8718021a1e34)