sig: Move runtime signal count check to compile time

Since the value of number_of_signals is known at compile time, we can
use a compile-time check instead. This also adds SIGLOST for the Hurd,
uses the correct signal counts for the Hurd and FreeBSD, and only gives
a compile-time warning when compiled on an unknown platform that it does
not know whether the number of signals is correct.
1 job for cross-platform in 1 minute and 54 seconds (queued for 6 seconds)
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Status Job ID Name Coverage
  Test
passed #36189946
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00:01:54