libgcrypt: fix compile error with '-O2' in sysroot path
Characters like '-O2' or '-Ofast' will be replaced by '-O1' when
compiling cipher. If we are cross compiling libgcrypt and sysroot
contains such characters, we would get compile errors because the
sysroot path has been modified.
Fix this by adding whitespaces before and after the original matching
pattern in the sed command.
