kboot: Implement symlink(2)

Linux's /dev/fd is implemented inside of /proc/self/fd, so we may need
to create a symlink to it early in boot. "/dev/fd" and "/dev/std*" might
not be strictly required for the boot loader, but should be present for
maximum flexibility.

Sponsored by:		Netflix

stand/kboot/arch/amd64/syscall_nr.h

@@ -12,6 +12,7 @@
 #define SYS_pselect6		270
 #define SYS_read		  0
 #define SYS_reboot		169
+#define SYS_symlinkat		266
 #define SYS_uname		 63
 #define SYS_write		  1
 

stand/kboot/arch/powerpc64/syscall_nr.h

@@ -13,6 +13,7 @@
 #define SYS_pselect6		280
 #define SYS_read		  3
 #define SYS_reboot		 88
+#define SYS_symlinkat		295
 #define SYS_uname		120
 #define SYS_write		  4
 

stand/kboot/host_syscall.h

@@ -68,6 +68,8 @@ typedef int64_t host_blkcnt_t;
 #define HOST_O_APPEND		02000
 #define HOST_O_NONBLOCK		04000
 
+#define HOST_AT_FDCWD		-100            /* Relative to current directory */
+
 /*
  * Data types
  */
@@ -104,6 +106,7 @@ int host_reboot(int, int, int, uintptr_t);
 int host_select(int nfds, long *readfds, long *writefds, long *exceptfds,
     struct host_timeval *timeout);
 int host_stat(const char *path, struct host_kstat *sb);
+int host_symlink(const char *path1, const char *path2);
 int host_uname(struct old_utsname *);
 ssize_t host_write(int fd, const void *buf, size_t nbyte);
 

stand/kboot/host_syscalls.c

@@ -110,6 +110,12 @@ host_stat(const char *path, struct host_kstat *sb)
 	return host_syscall(SYS_newfstatat, HOST_AT_FDCWD, (uintptr_t)path, (uintptr_t)sb, 0);
 }
 
+int
+host_symlink(const char *path1, const char *path2)
+{
+	return host_syscall(SYS_symlinkat, HOST_AT_FDCWD, path1, path2);
+}
+
 int
 host_uname(struct old_utsname *uts)
 {