Fix building and testing with the JS backend

Note that most tests have been disabled because they don't pass due
to missing C function support (e.g. pipe). But it's a good basis to
start adding tests for supported things (e.g. touchFd, cf #285).

This requires a recent version of GHC that provides
getMonotonicTimeNSec. See
https://gitlab.haskell.org/ghc/ghc/-/issues/23687 and
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/10900.

Author: hsyl20

System/Posix/Signals.hsc

@@ -112,7 +112,17 @@ import System.Posix.Process
 import System.Posix.Process.Internals
 import Data.Dynamic
 
+#if defined(javascript_HOST_ARCH)
+-- These signals aren't supported by the JS RTS
+-- and the header isn't exposed.
+##define STG_SIG_DFL   (-1)
+##define STG_SIG_IGN   (-2)
+##define STG_SIG_ERR   (-3)
+##define STG_SIG_HAN   (-4)
+##define STG_SIG_RST   (-5)
+#else
 ##include "rts/Signals.h"
+#endif
 
 import GHC.Conc hiding (Signal)