Fix ELF detection on Intel compilers

The Intel compiler does not define __amd64__ on x86_64 platforms;
instead, like other compilers, it defines __x86_64__ .
1 job for fix-libbsd-intel in 1 minute and 39 seconds (queued for 22 seconds)
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Status Job ID Name Coverage
  Test
passed #944353
test

00:01:39