[gdb] Use using in compat_x32_clock_t typedef

In commit 6c85ef111b0 ("[gdb] Use using instead of typedef"), I did:
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-typedef long __attribute__ ((__aligned__ (4))) compat_x32_clock_t;
+using compat_x32_clock_t = long __attribute__ ((__aligned__ (4)));
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which I had to revert because clang ignores the attribute in this variant.

Pedro suggested instead using:
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using compat_x32_clock_t [[gnu::aligned (4)]] = long;
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which does work with both clang and gcc.

[ Note that it's not a question of how the attribute is worded, this also works:
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using compat_x32_clock_t __attribute__ ((__aligned__ (4))) = long;
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It's just a question of where the attribute is placed. ]

Fix this by using Pedro's suggestion.

Suggested-By: Pedro Alves <pedro@palves.net>
Approved-By: Tom Tromey <tom@tromey.com>