[libc] Implement `timespec_get` (#116102)

`timespec_get` is C standard counterpart to POSIX `clock_gettime`. On
Linux we simply use `clock_gettime`. On baremetal we introduce a new
external API `_

[libc] Implement `timespec_get` (#116102)

`timespec_get` is C standard counterpart to POSIX `clock_gettime`. On
Linux we simply use `clock_gettime`. On baremetal we introduce a new
external API `__llvm_libc_timespec_get_utc` that should be implemented
by the vendor.

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