gcc: avoid excessive stat calls (#30005)

For about a decade GCC has an option `-f[no]-canonical-system-headers`
which basically runs `realpath` on all "system headers", to possibly
reduce the length of paths in diagnostics. [1]

Spack usually installs the "system headers" of GCC in very deeply nested
directories. Calling `realpath` there results in stat calls on every
level, for every header file. On some slow filesystem I have,
`-fno-canonical-system-headers` gives about 5x speedup to compile hello
world in C, meaning that ./configure scripts would be much faster when
using this flag by default.

[1] https://codereview.appspot.com/6495088

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