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Re #19416

This patch adds a flag to ghc allowing the parallelism of --make to be controlled with a POSIX semaphore, allowing parallel invocations of ghc --make to cooperate in limiting their CPU bound threads.

Several details remain to be decided, so this remains a draft:

• what should the flag be called? -jsem seems quite terrible
• An implementation for windows should be provided
• Should we instead(or as well) conform to the GNU make jobserver protocol? https://www.gnu.org/software/make/manual/html_node/Job-Slots.html
• Clients of GNU make jobserver are expected to have 1 active job, then claim additional jobs from the jobserver. This implementation just replaces the old QSem with a posix semaphore, so a ghc may have 0 active jobs. Is this Okay?

Building lens and it's dependencies from scratch on a 12 core machine shows some improvement:

without -jsem:

	Command being timed: "cabal --store-dir /home/doug/code/ghc.jsem/lenstest/_out/store v2-build lens --offline --ghc-options=-j -j"
	User time (seconds): 522.76
	System time (seconds): 44.70
	Percent of CPU this job got: 386%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 2:26.76

with -jsem:

	Command being timed: "cabal --store-dir /home/doug/code/ghc.jsem/lenstest/_out/store v2-build lens --offline --ghc-options=-j -jsem ghc.sem -j"
	User time (seconds): 416.23
	System time (seconds): 28.51
	Percent of CPU this job got: 335%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 2:12.51