Fix build with path names with 'align' or 'nop' in them.

clang is now inserting .file directives with the entire path in
them. This is fine, except that our sed peephole optimizer removes
them if ${SRCTOP} or ${OBJTOP} contains 'align' or 'nop', leading to
build failures. The sed peephole optimizer removes useful things for
boot2 when used with clang, so restrict its use to gcc. Also, gcc no
longer generates nops to pad things, so there's no point in removing
it. Specialize the optimization to just removing the .align 4 lines to
preclude inadvertant path matching.

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