[libc] Add a redirecting <syscall.h> header. (#191069)

The amount of legacy code including `<syscall.h>` header instead of
`<sys/syscall.h>` (which is the regular header location on Linux
systems) out there is large.

Add a simple one-liner redirecting header to fix this compatibility
issues. In this PR I omit the regular licensing blurb at the top, given
the transient nature of this file, but I'm happy to add this if needed.
Also, given that it's effectively a compatibility shim, YAML generation
is not used.

libc/config/linux/aarch64/headers.txt

@@ -53,6 +53,7 @@ set(TARGET_PUBLIC_HEADERS
     libc.include.sys_un
     libc.include.sys_utsname
     libc.include.sys_wait
+    libc.include.syscall
     libc.include.sysexits
     libc.include.termios
     libc.include.threads

libc/config/linux/riscv/headers.txt

@@ -52,6 +52,7 @@ set(TARGET_PUBLIC_HEADERS
     libc.include.sys_types
     libc.include.sys_utsname
     libc.include.sys_wait
+    libc.include.syscall
     libc.include.sysexits
     libc.include.termios
     libc.include.threads

libc/config/linux/x86_64/headers.txt

@@ -56,6 +56,7 @@ set(TARGET_PUBLIC_HEADERS
     libc.include.sys_un
     libc.include.sys_utsname
     libc.include.sys_wait
+    libc.include.syscall
     libc.include.sysexits
     libc.include.termios
     libc.include.threads

libc/include/CMakeLists.txt

@@ -180,6 +180,11 @@ add_header_macro(
     .llvm-libc-types.uint_ur_t
 )
 
+add_header(
+  syscall
+  HDR
+    syscall.h
+)
 
 add_header_macro(
   sysexits

libc/include/syscall.h

@@ -0,0 +1 @@
+#include <sys/syscall.h>