Fix typedef errors on platforms with _ASM_GENERIC_INT headers

[jrflat]

System fails to build on Amazon Linux 2 with the error:

/Swift/swift-system/Sources/CSystem/include/io_uring.h:141:18: error: typedef redefinition with different types ('uint64_t' (aka 'unsigned long') vs 'unsigned long long')
  141 | typedef uint64_t __u64;
      |                  ^
/usr/include/asm-generic/int-ll64.h:31:42: note: previous definition is here
   31 | __extension__ typedef unsigned long long __u64;
      |                                          ^

We can guard the __u64, __u32, etc. typedefs with #if !defined for the various _ASM_GENERIC_INT headers where these types are already defined.

Verified System builds successfully on Amazon Linux 2 with this change.

[jrflat]

@swift-ci please test