Make C++ mutex methods constant. #985

Job	Run time
flash-samples	38s
air-samples (4.0.5)	2m 58s
docs	1m 24s
air-samples (4.1.5)	2m 44s
android	27m 30s
air-samples (4.2.5)	2m 55s
air-samples (4.3.1)	2m 56s
html5-samples	1m 18s
ios	33m 35s
linux	13m 46s
macos	25m 11s
windows	15m 51s
package-haxelib	9m 13s
hashlink-samples (windows-latest)	5m 55s
hashlinkc-samples (windows-latest)	6m 51s
hashlink-samples (ubuntu-20.04)	6m 29s
neko-samples (3.4.7, windows-latest)	6m 53s
hashlinkc-samples (ubuntu-20.04)	6m 40s
hashlink-samples (macos-12)	7m 51s
neko-samples (3.4.7, ubuntu-20.04)	1m 27s
hashlinkc-samples (macos-12)	4m 8s
neko-samples (3.4.7, macos-12)	6m 53s
neko-samples (4.2.5, windows-latest)	6m 51s
neko-samples (4.2.5, ubuntu-20.04)	1m 1s
neko-samples (4.2.5, macos-12)	7m 50s
notify	3s
	3h 28m 51s

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