uapi headers: simplify the <linux/compiler.h> stuff.

We don't really need <linux/compiler.h> and <linux/compiler_types.h>. We
already have a mechanism to remove unused macros, so let's do that. We
don't currently have a way to remove unused #includes, so we still need
<linux/compiler.h> and <linux/compiler_types.h> files (but I've clarified
the comments in them).

I've kept the empty definitions of `__user` and `__force` for source
compatibility. (We had one security test at least that was assuming
a kernel struct definition will "just work".)

Bug: http://b/262917450
Test: treehugger
Change-Id: Iacbbbc1aeef9a4fac52dabd7811ab875cc267d4f
diff --git a/libc/kernel/uapi/linux/fs.h b/libc/kernel/uapi/linux/fs.h
index 96f9181..b8bfaac 100644
--- a/libc/kernel/uapi/linux/fs.h
+++ b/libc/kernel/uapi/linux/fs.h
@@ -190,10 +190,10 @@
 #define SYNC_FILE_RANGE_WAIT_AFTER 4
 #define SYNC_FILE_RANGE_WRITE_AND_WAIT (SYNC_FILE_RANGE_WRITE | SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WAIT_AFTER)
 typedef int __bitwise __kernel_rwf_t;
-#define RWF_HIPRI ((__force __kernel_rwf_t) 0x00000001)
-#define RWF_DSYNC ((__force __kernel_rwf_t) 0x00000002)
-#define RWF_SYNC ((__force __kernel_rwf_t) 0x00000004)
-#define RWF_NOWAIT ((__force __kernel_rwf_t) 0x00000008)
-#define RWF_APPEND ((__force __kernel_rwf_t) 0x00000010)
+#define RWF_HIPRI (( __kernel_rwf_t) 0x00000001)
+#define RWF_DSYNC (( __kernel_rwf_t) 0x00000002)
+#define RWF_SYNC (( __kernel_rwf_t) 0x00000004)
+#define RWF_NOWAIT (( __kernel_rwf_t) 0x00000008)
+#define RWF_APPEND (( __kernel_rwf_t) 0x00000010)
 #define RWF_SUPPORTED (RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT | RWF_APPEND)
 #endif