Revise pthread_condattr_setclock on Android

[MichalStrehovsky]

See #58701 (comment)

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We can likely use pthread_condattr_setclock on Android in NDK r21. Also our minimum supported ios version is now greater than the version that didn't support CLOCK_MONOTONIC.

So we can likely change the ifdef around BROKEN_CLOCK_SOURCE to exclude ios and Android

runtime/src/mono/mono/utils/mono-os-mutex.h

Lines 39 to 41 in e55968b

	#if !defined(CLOCK_MONOTONIC) || defined(HOST_DARWIN) || defined(HOST_ANDROID) || defined(HOST_WASM)
	#define BROKEN_CLOCK_SOURCE
	#endif

[filipnavara]

I think it's broken or unavailable on API level <= 20, which would roughly mean that it is available since Android 5.0. What is the minimum supported version now?

[MichalStrehovsky]

runtime/src/mono/Directory.Build.props

Line 32 in 57bfe47

<AndroidApiVersion>21</AndroidApiVersion>

[MichalStrehovsky]

The original pull request got reverted because it fails a GCC leg that doesn't run for this type of changes - #58744.

[lambdageek]

I pushed this to 7.0, feel free to change back to 6.0 if you feel like it should go earlier.
The actual change is trivial, but I'm not sure if we will recognize failures if it leads to problems.