clock_gettime is a POSIX API that may not be available on platforms like
baremetal; timespec_get is the C11 equivalent. This change adds support
for using timespec_get instead of clock_gettime to improve compatibility
with non-POSIX platforms. For now, this is only enabled with LLVM libc
which implemented timespec_get in #116102, but in the future this can be
expanded to other platforms.
Related to #84879.