[libc] Fix missing sysroot path for kernel headers when crosscompiling (#99588)

When crosscompiling, we need to search for the linux kernel headers in the sysroot but since #97486 the linux kernel headers were always searched in /usr/include.

This patch fixes this behaviour by prepending a '=' to where we search for the kernel headers. As per the gcc/clang's documentation a '=' before the path is replaced by the sysroot.

This patch also includes a fix for rv32, that fails to compile due to a missing definition of CLOCK_REALTIME after this change.

Author: mikhailramalho

libc/cmake/modules/LLVMLibCCompileOptionRules.cmake

@@ -88,7 +88,11 @@ function(_get_common_compile_options output_var flags)
          (LIBC_CC_SUPPORTS_NOSTDLIBINC OR COMPILER_RESOURCE_DIR))
         # We use -idirafter to avoid preempting libc's own headers in case the
         # directory (e.g. /usr/include) contains other headers.
-        list(APPEND compile_options "-idirafter${LIBC_KERNEL_HEADERS}")
+        if(CMAKE_CROSSCOMPILING)
+          list(APPEND compile_options "-idirafter=${LIBC_KERNEL_HEADERS}")
+        else()
+          list(APPEND compile_options "-idirafter${LIBC_KERNEL_HEADERS}")
+        endif()
       endif()
     endif()
 

libc/src/time/linux/nanosleep.cpp

@@ -7,7 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/time/nanosleep.h"
-
+#include "hdr/time_macros.h"
 #include "src/__support/OSUtil/syscall.h" // For syscall functions.
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"