Broken build on 32 bit: Fix -Wshorten-64-to-32.

Problem:

[ 48%] Building C object src/CMakeFiles/nvim.dir/os/mem.c.o
/Users/eliseo/projects/os/neovim/src/os/mem.c:9:32: error: implicit
conversion loses integer
      precision: 'uint64_t' (aka 'unsigned long long') to 'long_u' (aka
      'unsigned long')
            [-Werror,-Wshorten-64-to-32]
              return uv_get_total_memory() >> 10;
                ~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~^~~~~

Solution:

Avoid conversion. Make function return proper uint64_t.
Make users of the function accomodate the value if too big for them.

src/option.c

@@ -2011,8 +2011,11 @@ void set_init_1(void)
 #endif
     {
 #ifdef HAVE_TOTAL_MEM
-      /* Use amount of memory available to Vim. */
+      /* Use half of amount of memory available to Vim. */
-      n = (os_get_total_mem_kib() >> 1);
+      /* If too much to fit in long_u, get long_u max */
+      uint64_t available_kib = os_get_total_mem_kib();
+      n = available_kib / 2 > ULONG_MAX ? ULONG_MAX
+                                        : (long_u)(available_kib /2);
 #else
       n = (0x7fffffff >> 11);
 #endif

src/os/mem.c

@@ -4,7 +4,7 @@
 
 #include "os/os.h"
 
-long_u os_get_total_mem_kib(void) {
+uint64_t os_get_total_mem_kib(void) {
   // Convert bytes to KiB.
   return uv_get_total_memory() >> 10;
 }

src/os/os.h

@@ -79,7 +79,7 @@ int os_rmdir(const char *path);
 int os_remove(const char *path);
 
 /// Get the total system physical memory in KiB.
-long_u os_get_total_mem_kib(void);
+uint64_t os_get_total_mem_kib(void);
 const char *os_getenv(const char *name);
 int os_setenv(const char *name, const char *value, int overwrite);
 char *os_getenvname_at_index(size_t index);