Make improvements

- More timspec_*() and timeval_*() APIs have been introduced.
- The copyfd() function is now simplified thanks to POSIX rules.
- More Cosmo-specific APIs have been moved behind the COSMO define.
- The setitimer() polyfill for Windows NT is now much higher quality.
- Fixed build error for MODE=aarch64 due to -mstringop-strategy=loop.
- This change introduces `make MODE=nox87 toolchain` which makes it
  possible to build programs using your cosmocc toolchain that don't
  have legacy fpu instructions. This is useful, for example, if you
  want to have a ~22kb tinier blink virtual machine.

libc/tinymath/round.c

@@ -92,5 +92,5 @@ double round(double x)
 }
 
 #if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
-__strong_reference(round, roundl);
+__weak_reference(round, roundl);
 #endif